The following packages have unmet dependencies:

Are you aware that Ubuntu 20.04 (focal) is still in development and not yet stable?

For diagnostic purposes please provide the output of the following commands:

uname -a
lsb_release -crid
sudo dpkg --audit
apt-cache policy libqtermwidget5-0 qtermwidget5-data
apt --simulate install -f

It is Lubuntu to clarify at the outset.

Yes. I know I am running development build of Lubuntu 20.04.
----------------------------------------
$ uname -a
Linux user-pc 5.4.0-14-generic #17-Ubuntu SMP Thu Feb 6 22:47:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

------------------------------------------------------
$ lsb_release -crid
Distributor ID: Ubuntu
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04
Codename: focal
--------------------------------------------------
$ sudo dpkg --audit

he following packages have been unpacked but not yet configured.
They must be configured using dpkg --configure or the configure
menu option in dselect for them to work:
 apparmor user-space parser utility for AppArmor
 bluedevil KDE Bluetooth stack
 breeze-cursor-theme Default Plasma cursor theme.
 colord system service to manage device colour profiles -- system
 colord-data system service to manage device colour profiles -- data f
 cups Common UNIX Printing System(tm) - PPD/driver support, web
 cups-browsed OpenPrinting CUPS Filters - cups-browsed
 cups-bsd Common UNIX Printing System(tm) - BSD commands
 cups-client Common UNIX Printing System(tm) - client programs (SysV)
 cups-common Common UNIX Printing System(tm) - common files
 cups-core-drivers Common UNIX Printing System(tm) - driverless printing
 cups-daemon Common UNIX Printing System(tm) - daemon
 cups-filters OpenPrinting CUPS Filters - Main Package
 cups-filters-core-drivers OpenPrinting CUPS Filters - Driverless printing
 cups-ipp-utils Common UNIX Printing System(tm) - IPP developer/admin uti
 cups-ppdc Common UNIX Printing System(tm) - PPD manipulation utilit
 cups-server-common Common UNIX Printing System(tm) - server common files
 fdisk collection of partitioning utilities
 firefox Safe and easy web browser from Mozilla
 genisoimage Creates ISO-9660 CD-ROM filesystem images
 gnome-disk-utility manage and configure disk drives and media
 initramfs-tools generic modular initramfs generator (automation)
 initramfs-tools-bin binaries used by initramfs-tools
 initramfs-tools-core generic modular initramfs generator (core tools)
 ippusbxd Daemon for IPP USB printer support
 kactivitymanagerd System service to manage user's activities
 kde-cli-tools tools to use KDE services from the command line
 kde-cli-tools-data tools to use kioslaves from the command line
 kde-style-breeze Widget style for Qt and KDE Software
 kwayland-integration:amd64 kwayland runtime integration plugins
 language-selector-common Language selector for Ubuntu
 libapparmor1:amd64 changehat AppArmor library
 libbrotli1:amd64 library implementing brotli encoder and decoder (shared l
 libbrotli1:i386 library implementing brotli encoder and decoder (shared l
 libcolord2:amd64 system service to manage device colour profiles -- runtim
 libcolorhug2:amd64 library to access the ColorHug colourimeter -- runtime
 libcups2:amd64 Common UNIX Printing System(tm) - Core library
 libcups2:i386 Common UNIX Printing System(tm) - Core library
 libcupsfilters1:amd64 OpenPrinting CUPS Filters - Shared library
 libcupsimage2:amd64 Common UNIX Printing System(tm) - Raster image library
 libfdisk1:amd64 fdisk partitioning library
 libfontembed1:amd64 OpenPrinting CUPS Filters - Font Embed Shared library
 libio-socket-ssl-perl Perl module implementing object oriented interface to SS
 libkf5screen-bin library for screen management - helpers
 libkf5screen7:amd64 library for screen management - shared library
 libkworkspace5-5 Plasma Workspace for KF5 library
 libnewt0.52:amd64 Not Erik's Windowing Toolkit - text mode windowing with s
 libpython3-stdlib:amd64 interactive high-level object-oriented language (default
 libqtermwidget5-0:amd64 Terminal emulator widget for Qt 5 (shared libraries)
 libsndfile1:amd64 Library for reading/writing audio files
 libsndfile1:i386 Library for reading/writing audio files
 lubuntu-default-settings default settings for Lubuntu
 mount tools for mounting and manipulating filesystems
 plasma-discover Discover software management suite
 plasma-discover-common Discover software manager suite (common data files)
 polkit-kde-agent-1 KDE dialogs for PolicyKit
 python3 interactive high-level object-oriented language (default
 python3-cairo:amd64 Python3 bindings for the Cairo vector graphics library
 python3-gi Python 3 bindings for gobject-introspection libraries
 python3-renderpm:amd64 python low level render interface
 python3-reportlab ReportLab library to create PDF documents using Python3
 python3-reportlab-accel:amd64 C coded extension accelerator for the ReportLab Toolki
 python3-sip Python 3/C++ bindings generator runtime library
 python3-systemd Python 3 bindings for systemd
 rfkill tool for enabling and disabling wireless devices
 uuid-runtime runtime components for the Universally Unique ID library
 vim-common Vi IMproved - Common files
 vim-tiny Vi IMproved - enhanced vi editor - compact version
 whiptail Displays user-friendly dialog boxes from shell scripts
 xxd tool to make (or reverse) a hex dump

The following packages have been triggered, but the trigger processing
has not yet been done. Trigger processing can be requested using
dselect or dpkg --configure --pending (or dpkg --triggers-only):
 dbus simple interprocess messaging system (daemon and utilitie
 desktop-file-utils Utilities for .desktop files
 hicolor-icon-theme default fallback theme for FreeDesktop.org icon themes
 libc-bin GNU C Library: Binaries
 libglib2.0-0:amd64 GLib library of C routines
 libglib2.0-0:i386 GLib library of C routines
 man-db tools for reading manual pages
 mime-support MIME files 'mime.types' & 'mailcap', and support programs
 shared-mime-info FreeDesktop.org shared MIME database and spec
 systemd system and service manager
 ufw program for managing a Netfilter firewall
--------------------------------------------
$ apt-cache policy libqtermwidget5-0 qtermwidget5-data
libqtermwidget5-0:
  Installed: 0.14.1-2
  Candidate: 0.14.1-2
  Version table:
 *** 0.14.1-2 500
        500 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status
qtermwidget5-data:
  Installed: 0.14.1-0ubuntu3
  Candidate: 0.14.1-2
  Version table:
     0.14.1-2 500
        500 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu focal/universe i386 Packages
 *** 0.14.1-0ubuntu3 100
        100 /var/lib/dpkg/status
------------------------------------------------
$ apt --simulate install -f
NOTE: This is only a simulation!
      apt needs root privileges for real execution.
      Keep also in mind that locking is deactivated,
      so don't depend on the relevance to the real current situation!
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... Done
The following package was automatically installed and is no longer required:
  fonts-hack-ttf
Use 'apt autoremove' to remove it.
The following additional packages will be installed:
  qtermwidget5-data
The following packages will be upgraded:
  qtermwidget5-data
1 upgraded, 0 newly installed, 0 to remove and 129 not upgraded.
70 not fully installed or removed.
Inst qtermwidget5-data [0.14.1-0ubuntu3] (0.14.1-2 Ubuntu:20.04/focal [all])
Conf qtermwidget5-data (0.14.1-2 Ubuntu:20.04/focal [all])
Conf fdisk (2.34-0.1ubuntu7 Ubuntu:20.04/focal [amd64])
Conf libcups2 (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf libcups2:i386 (2.3.1-7 Ubuntu:20.04/focal [i386])
Conf uuid-runtime (2.34-0.1ubuntu7 Ubuntu:20.04/focal [amd64])
Conf libfdisk1 (2.34-0.1ubuntu7 Ubuntu:20.04/focal [amd64])
Conf kde-cli-tools (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf vim-common (2:8.1.2269-1ubuntu1 Ubuntu:20.04/focal [all])
Conf lubuntu-default-settings (20.04.4 Ubuntu:20.04/focal [all])
Conf bluedevil (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf breeze-cursor-theme (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [all])
Conf libbrotli1 (1.0.7-6build1 Ubuntu:20.04/focal [amd64])
Conf libbrotli1:i386 (1.0.7-6build1 Ubuntu:20.04/focal [i386])
Conf python3-systemd (234-3build2 Ubuntu:20.04/focal [amd64])
Conf language-selector-common (0.202 Ubuntu:20.04/focal [all])
Conf plasma-discover-common (5.18.1-0ubuntu1 Ubuntu:20.04/focal [all])
Conf python3-reportlab-accel (3.5.34-1build1 Ubuntu:20.04/focal [amd64])
Conf kde-style-breeze (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf plasma-discover (5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf python3 (3.8.0-3ubuntu1 Ubuntu:20.04/focal [amd64])
Conf cups-server-common (2.3.1-7 Ubuntu:20.04/focal [all])
Conf python3-sip (4.19.21+dfsg-1build1 Ubuntu:20.04/focal [amd64])
Conf kactivitymanagerd (5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf cups-common (2.3.1-7 Ubuntu:20.04/focal [all])
Conf initramfs-tools-bin (0.136ubuntu1 Ubuntu:20.04/focal [amd64])
Conf colord-data (1.4.4-1ubuntu2 Ubuntu:20.04/focal [all])
Conf libapparmor1 (2.13.3-7ubuntu2 Ubuntu:20.04/focal [amd64])
Conf libio-socket-ssl-perl (2.067-1 Ubuntu:20.04/focal [all])
Conf gnome-disk-utility (3.35.91-1ubuntu1 Ubuntu:20.04/focal [amd64])
Conf libkworkspace5-5 (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf python3-renderpm (3.5.34-1build1 Ubuntu:20.04/focal [amd64])
Conf mount (2.34-0.1ubuntu7 Ubuntu:20.04/focal [amd64])
Conf kde-cli-tools-data (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [all])
Conf cups-filters (1.27.1-2 Ubuntu:20.04/focal [amd64])
Conf cups-ppdc (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf kwayland-integration (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf whiptail (0.52.21-4ubuntu2 Ubuntu:20.04/focal [amd64])
Conf libnewt0.52 (0.52.21-4ubuntu2 Ubuntu:20.04/focal [amd64])
Conf libpython3-stdlib (3.8.0-3ubuntu1 Ubuntu:20.04/focal [amd64])
Conf libcupsfilters1 (1.27.1-2 Ubuntu:20.04/focal [amd64])
Conf libkf5screen-bin (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf libkf5screen7 (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf libsndfile1 (1.0.28-7 Ubuntu:20.04/focal [amd64])
Conf libsndfile1:i386 (1.0.28-7 Ubuntu:20.04/focal [i386])
Conf libcolord2 (1.4.4-1ubuntu2 Ubuntu:20.04/focal [amd64])
Conf rfkill (2.34-0.1ubuntu7 Ubuntu:20.04/focal [amd64])
Conf libfontembed1 (1.27.1-2 Ubuntu:20.04/focal [amd64])
Conf xxd (2:8.1.2269-1ubuntu1 Ubuntu:20.04/focal [amd64])
Conf genisoimage (9:1.1.11-3.1ubuntu1 Ubuntu:20.04/focal [amd64])
Conf cups-filters-core-drivers (1.27.1-2 Ubuntu:20.04/focal [amd64])
Conf python3-cairo (1.16.2-2ubuntu2 Ubuntu:20.04/focal [amd64])
Conf python3-gi (3.34.0-6build1 Ubuntu:20.04/focal [amd64])
Conf libqtermwidget5-0 (0.14.1-2 Ubuntu:20.04/focal [amd64])
Conf firefox (73.0.1+build1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf vim-tiny (2:8.1.2269-1ubuntu1 Ubuntu:20.04/focal [amd64])
Conf apparmor (2.13.3-7ubuntu2 Ubuntu:20.04/focal [amd64])
Conf cups-bsd (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf cups-core-drivers (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf cups-daemon (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf polkit-kde-agent-1 (4:5.18.1-0ubuntu1 Ubuntu:20.04/focal [amd64])
Conf libcolorhug2 (1.4.4-1ubuntu2 Ubuntu:20.04/focal [amd64])
Conf python3-reportlab (3.5.34-1build1 Ubuntu:20.04/focal [all])
Conf libcupsimage2 (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf cups (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf cups-client (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf cups-ipp-utils (2.3.1-7 Ubuntu:20.04/focal [amd64])
Conf initramfs-tools-core (0.136ubuntu1 Ubuntu:20.04/focal [all])
Conf initramfs-tools (0.136ubuntu1 Ubuntu:20.04/focal [all])
Conf ippusbxd (1.34-2ubuntu1 Ubuntu:20.04/focal [amd64])
Conf cups-browsed (1.27.1-2 Ubuntu:20.04/focal [amd64])
Conf colord (1.4.4-1ubuntu2 Ubuntu:20.04/focal [amd64])

Just in case if it is of any help to you.

 I think I ran

sudo apt dist-upgrade

before getting to this trouble.

Neither "sudo apt upgrade" nor "sudo apt dist-upgrade" should lead to this situation.
The output of the last of the commands let me expect that the command

sudo apt install -f

will correct the problem.
Try this command and report the results.

That does not solve the problem.

$ sudo apt install -f
[sudo] password for user:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... Done
The following package was automatically installed and is no longer required:
  fonts-hack-ttf
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  qtermwidget5-data
The following packages will be upgraded:
  qtermwidget5-data
1 upgraded, 0 newly installed, 0 to remove and 129 not upgraded.
70 not fully installed or removed.
Need to get 0 B/19.8 kB of archives.
After this operation, 54.3 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
(Reading database ... 257182 files and directories currently installed.)
Preparing to unpack .../qtermwidget5-data_0.14.1-2_all.deb ...
Unpacking qtermwidget5-data (0.14.1-2) over (0.14.1-0ubuntu3) ...
dpkg: error processing archive /var/cache/apt/archives/qtermwidget5-data_0.14.1-2_all.deb (--unpack):
 trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_ca.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
Errors were encountered while processing:
 /var/cache/apt/archives/qtermwidget5-data_0.14.1-2_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

Ok, now there is a significant and useful error message.

What output do you receive for the command

apt --simulate purge qtermwidget-l10n

$ sudo apt --simulate purge qtermwidget-l10n

[sudo] password for user:
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 libqtermwidget5-0 : Depends: qtermwidget5-data (= 0.14.1-2) but 0.14.1-0ubuntu3 is to be installed
                     Recommends: qtermwidget-l10n but it is not going to be installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

I wrongly clicked solved button

That command did not work, what output do you receive for

apt --simulate purge qtermwidget-l10n -f

$ sudo apt --simulate purge qtermwidget-l10n -f

Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 libqtermwidget5-0 : Depends: qtermwidget5-data (= 0.14.1-2) but 0.14.1-0ubuntu3 is to be installed
                     Recommends: qtermwidget-l10n but it is not going to be installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

$ sudo apt install qtermwidget-l10n- -f
[sudo] password for user:
Reading package lists... Done
Building dependency tree
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 libqtermwidget5-0 : Depends: qtermwidget5-data (= 0.14.1-2) but 0.14.1-0ubuntu3 is to be installed
                     Recommends: qtermwidget-l10n but it is not going to be installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

$ sudo apt install qtermwidget-l10n- qtermwidget5-data libqtermwidget5-0 -f

Reading package lists... Done
Building dependency tree
Reading state information... Done
libqtermwidget5-0 is already the newest version (0.14.1-2).
libqtermwidget5-0 set to manually installed.
The following package was automatically installed and is no longer required:
  fonts-hack-ttf
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  qtermwidget-l10n
The following packages will be upgraded:
  qtermwidget5-data
1 upgraded, 0 newly installed, 1 to remove and 129 not upgraded.
70 not fully installed or removed.
Need to get 0 B/19.8 kB of archives.
After this operation, 14.3 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 257182 files and directories currently installed.)
Preparing to unpack .../qtermwidget5-data_0.14.1-2_all.deb ...
Unpacking qtermwidget5-data (0.14.1-2) over (0.14.1-0ubuntu3) ...
dpkg: error processing archive /var/cache/apt/archives/qtermwidget5-data_0.14.1-2_all.deb (--unpack):
 trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_ca.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
Errors were encountered while processing:
 /var/cache/apt/archives/qtermwidget5-data_0.14.1-2_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

$ sudo apt -o Dpkg::Options::="--force-overwrite" install -f
Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... Done
The following package was automatically installed and is no longer required:
  fonts-hack-ttf
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  qtermwidget5-data
The following packages will be upgraded:
  qtermwidget5-data
1 upgraded, 0 newly installed, 0 to remove and 129 not upgraded.
70 not fully installed or removed.
Need to get 0 B/19.8 kB of archives.
After this operation, 54.3 kB of additional disk space will be used.
Do you want to continue? [Y/n]
(Reading database ... 257182 files and directories currently installed.)
Preparing to unpack .../qtermwidget5-data_0.14.1-2_all.deb ...
Unpacking qtermwidget5-data (0.14.1-2) over (0.14.1-0ubuntu3) ...
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_ca.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_cs.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_cy.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_da.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_de.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_el.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_es.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_fr.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_gl.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_he.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_hu.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_ja.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_lt.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_nb_NO.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_pl.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_pt.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_tr.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_zh_CN.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
dpkg: warning: overriding problem because --force enabled:
dpkg: warning: trying to overwrite '/usr/share/qtermwidget5/translations/qtermwidget_zh_TW.qm', which is also in package qtermwidget-l10n 0.14.1-0ubuntu3
Setting up genisoimage (9:1.1.11-3.1ubuntu1) ...
Setting up kde-style-breeze (4:5.18.1-0ubuntu1) ...
Setting up libapparmor1:amd64 (2.13.3-7ubuntu2) ...
Setting up libnewt0.52:amd64 (0.52.21-4ubuntu2) ...
Setting up libkworkspace5-5 (4:5.18.1-0ubuntu1) ...
Setting up cups-server-common (2.3.1-7) ...
Setting up firefox (73.0.1+build1-0ubuntu1) ...
Please restart all running instances of firefox, or you will experience problems.
Setting up libfontembed1:amd64 (1.27.1-1) ...
Setting up breeze-cursor-theme (4:5.18.1-0ubuntu1) ...
Setting up libkf5screen7:amd64 (4:5.18.1-0ubuntu1) ...
Setting up kactivitymanagerd (5.18.1-0ubuntu1) ...
Setting up cups-common (2.3.1-7) ...
Setting up libbrotli1:amd64 (1.0.7-6build1) ...
Setting up libbrotli1:i386 (1.0.7-6build1) ...
Setting up lubuntu-default-settings (20.04.4) ...
Installing new version of config file /etc/xdg/xdg-Lubuntu/openbox/lxqt-rc.xml ...
Setting up libcolord2:amd64 (1.4.4-1ubuntu2) ...
Setting up xxd (2:8.1.2269-1ubuntu1) ...
Setting up colord-data (1.4.4-1ubuntu2) ...
Setting up vim-common (2:8.1.2269-1ubuntu1) ...

Configuration file '/etc/vim/vimrc'
 ==> Deleted (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** vimrc (Y/I/N/O/D/Z) [default=N] ?
Setting up rfkill (2.34-0.1ubuntu6) ...
Setting up polkit-kde-agent-1 (4:5.18.1-0ubuntu1) ...
Setting up qtermwidget5-data (0.14.1-2) ...
Setting up libfdisk1:amd64 (2.34-0.1ubuntu6) ...
Setting up libcups2:amd64 (2.3.1-7) ...
Setting up libcups2:i386 (2.3.1-7) ...
Setting up whiptail (0.52.21-4ubuntu2) ...
Setting up mount (2.34-0.1ubuntu6) ...
Setting up uuid-runtime (2.34-0.1ubuntu6) ...
Setting up bluedevil (4:5.18.1-0ubuntu1) ...
Setting up gnome-disk-utility (3.35.91-1ubuntu1) ...
Setting up libsndfile1:amd64 (1.0.28-7) ...
Setting up libsndfile1:i386 (1.0.28-7) ...
Setting up kde-cli-tools-data (4:5.18.1-0ubuntu1) ...
Setting up libcupsfilters1:amd64 (1.27.1-1) ...
Setting up libio-socket-ssl-perl (2.067-1) ...
Setting up libpython3-stdlib:amd64 (3.8.0-3ubuntu1) ...
Setting up kwayland-integration:amd64 (4:5.18.1-0ubuntu1) ...
Setting up initramfs-tools-bin (0.136ubuntu1) ...
Setting up plasma-discover-common (5.18.1-0ubuntu1) ...
Setting up cups-ipp-utils (2.3.1-7) ...
Setting up libkf5screen-bin (4:5.18.1-0ubuntu1) ...
Setting up cups-ppdc (2.3.1-7) ...
Setting up kde-cli-tools (4:5.18.1-0ubuntu1) ...
Setting up libcolorhug2:amd64 (1.4.4-1ubuntu2) ...
Setting up python3 (3.8.0-3ubuntu1) ...
running python rtupdate hooks for python3.8...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
/usr/share/hplip/base/utils.py:2060: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if weburl is "" or weburl is None:
/usr/share/hplip/check-plugin.py:116: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if log_level is 'debug':
/usr/share/hplip/check.py:685: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if 'getfacl' not in g and '' is not g and 'file' not in g:
/usr/share/hplip/installer/core_install.py:2037: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if home_dir is "":
/usr/share/hplip/ui5/devmgr_ext.py:15: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self.latest_available_version is not "":
/usr/share/hplip/ui5/devmgr_ext.py:37: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self.latest_available_version is not "":
/usr/bin/py3compile:256: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
  log.warn('No such file or directory: %s', item)
W: py3compile:256: No such file or directory: /usr/lib/lubuntu-update-notifier
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
running python post-rtupdate hooks for python3.8...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up vim-tiny (2:8.1.2269-1ubuntu1) ...
Setting up cups-filters-core-drivers (1.27.1-1) ...
Setting up cups-client (2.3.1-7) ...
Setting up fdisk (2.34-0.1ubuntu6) ...
Setting up libcupsimage2:amd64 (2.3.1-7) ...
Setting up libqtermwidget5-0:amd64 (0.14.1-2) ...
Setting up apparmor (2.13.3-7ubuntu2) ...
Reloading AppArmor profiles
Skipping profile in /etc/apparmor.d/disable: usr.bin.firefox
AppArmor parser error for /etc/apparmor.d in /etc/apparmor.d/usr.lib.snapd.snap-confine.real at line 11: Could not open '/var/lib/snapd/apparmor/snap-confine'
Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
Skipping profile in /etc/apparmor.d/disable: usr.bin.firefox
AppArmor parser error for /etc/apparmor.d/usr.lib.snapd.snap-confine.real in /etc/apparmor.d/usr.lib.snapd.snap-confine.real at line 11: Could not open '/var/lib/snapd/apparmor/snap-confine'
Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
Error: At least one profile failed to load
AppArmor parser error for /var/lib/snapd/apparmor/profiles in /var/lib/snapd/apparmor/profiles/snap-confine.snapd.5754 at line 11: Could not open '/var/lib/snapd/apparmor/snap-confine'
AppArmor parser error for /var/lib/snapd/apparmor/profiles/snap-confine.snapd.5754 in /var/lib/snapd/apparmor/profiles/snap-confine.snapd.5754 at line 11: Could not open '/var/lib/snapd/apparmor/snap-confine'
Error: At least one profile failed to load
Setting up ippusbxd (1.34-2) ...
Installing new version of config file /etc/apparmor.d/usr.sbin.ippusbxd ...
Setting up plasma-discover (5.18.1-0ubuntu1) ...
Setting up python3-gi (3.34.0-6build1) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up python3-reportlab-accel:amd64 (3.5.34-1build1) ...
Setting up cups-daemon (2.3.1-7) ...
Setting up python3-reportlab (3.5.34-1build1) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up language-selector-common (0.202) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up cups-filters (1.27.1-1) ...
Setting up initramfs-tools-core (0.136ubuntu1) ...
Setting up python3-systemd (234-3build2) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up python3-sip (4.19.21+dfsg-1build1) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up cups-browsed (1.27.1-1) ...
Setting up cups-core-drivers (2.3.1-7) ...
Setting up python3-cairo:amd64 (1.16.2-2ubuntu2) ...
/usr/lib/python3.8/subprocess.py:838: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.stdin = io.open(p2cwrite, 'wb', bufsize)
Setting up python3-renderpm:amd64 (3.5.34-1build1) ...
Setting up initramfs-tools (0.136ubuntu1) ...
update-initramfs: deferring update (trigger activated)
Setting up cups (2.3.1-7) ...
Updating PPD files for cups ...
Updating PPD files for cups-filters ...
Setting up cups-bsd (2.3.1-7) ...
Setting up colord (1.4.4-1ubuntu2) ...
^[[B^[[BProcessing triggers for ufw (0.36-2~ubuntu2) ...
Processing triggers for systemd (244.2-1ubuntu1) ...
Processing triggers for man-db (2.9.0-2) ...
Processing triggers for dbus (1.12.16-2ubuntu2) ...
Processing triggers for shared-mime-info (1.10-1) ...
 Processing triggers for desktop-file-utils (0.24-1ubuntu1) ...
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for libglib2.0-0:i386 (2.63.3-3) ...
Processing triggers for libglib2.0-0:amd64 (2.63.3-3) ...
Processing triggers for libc-bin (2.30-0ubuntu3) ...
Processing triggers for initramfs-tools (0.136ubuntu1) ...
update-initramfs: Generating /boot/initrd.img-5.4.0-14-generic

Now

sudo apt update followed by
sudo apt upgrade

works!

Thanks Manfred Hampl, that solved my question.

ok, using the -force option seems to have worked, but to make sure that everything is in a fully clean status, you should execute the following commands:

sudo apt purge qtermwidget-l10n
sudo apt install --reinstall qtermwidget5-data
sudo apt-mark auto libqtermwidget5-0

and finally install all available updates with

sudo apt full-upgrade

(if you haven't already done this).

Well.

I ran update and upgrade before I saw your above latest comment.

Anyhow, ran them after update upgrade cycle.
---------
$ sudo apt purge qtermwidget-l10n
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  qtermwidget-l10n*
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 68.6 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 256549 files and directories currently installed.)
Removing qtermwidget-l10n (0.14.1-0ubuntu3) ...
------------

$ sudo apt install --reinstall qtermwidget5-data
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 19.8 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu focal/universe amd64 qtermwidget5-data all 0.14.1-2 [19.8 kB]
Fetched 19.8 kB in 1s (13.9 kB/s)
(Reading database ... 256548 files and directories currently installed.)
Preparing to unpack .../qtermwidget5-data_0.14.1-2_all.deb ...
Unpacking qtermwidget5-data (0.14.1-2) over (0.14.1-2) ...
Setting up qtermwidget5-data (0.14.1-2) ...

-------------------------
$ sudo apt-mark auto libqtermwidget5-0

libqtermwidget5-0 set to automatically installed.

Thanks.
But why this happened?
Was not apt battle-hardened not to allow this?

This issue is already under investigation, Bug #1864170

Why this happened? Probably human error.