Ubuntu Software error: "unable to install..."

Just to add that the error occurs with many packages, but not all. For example, I was able to install Synaptic from Ubuntu Software. But I got the error with Nextcloud, with TOR, with Snap Store Proxy Client

For diagnostic purposes please provide the output that you receive for the following commands (to be executed in a terminal window):

uname -a
lsb_release -crid
sudo apt-get update
sudo dpkg --audit
sudo dpkg --configure -a

paul@paul-ThinkPad-T420s:~$ uname -a
Linux paul-ThinkPad-T420s 4.15.0-39-generic #42-Ubuntu SMP Tue Oct 23 15:48:01 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
paul@paul-ThinkPad-T420s:~$ lsb_release -crid
Distributor ID: Ubuntu
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic
paul@paul-ThinkPad-T420s:~$ sudo apt-get update
[sudo] password for paul:
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Hit:3 http://dl.google.com/linux/chrome/deb stable Release
Hit:4 http://linux.teamviewer.com/deb stable InRelease
Hit:5 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Hit:6 http://archive.canonical.com/ubuntu bionic InRelease
Hit:7 http://ppa.launchpad.net/atareao/atareao/ubuntu bionic InRelease
Get:9 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Hit:10 http://ppa.launchpad.net/nextcloud-devs/client/ubuntu bionic InRelease
Get:11 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:12 http://security.ubuntu.com/ubuntu bionic-security/main amd64 DEP-11 Metadata [204 B]
Get:13 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 DEP-11 Metadata [10.6 kB]
Get:14 http://security.ubuntu.com/ubuntu bionic-security/universe DEP-11 48x48 Icons [9,088 B]
Get:15 http://security.ubuntu.com/ubuntu bionic-security/universe DEP-11 64x64 Icons [20.1 kB]
Get:16 http://us.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages [394 kB]
Get:17 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [444 kB]
Get:18 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 DEP-11 Metadata [233 kB]
Get:19 http://us.archive.ubuntu.com/ubuntu bionic-updates/main DEP-11 48x48 Icons [52.2 kB]
Get:20 http://us.archive.ubuntu.com/ubuntu bionic-updates/main DEP-11 64x64 Icons [99.1 kB]
Get:21 http://us.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 DEP-11 Metadata [195 kB]
Get:22 http://us.archive.ubuntu.com/ubuntu bionic-updates/universe DEP-11 48x48 Icons [175 kB]
Get:23 http://us.archive.ubuntu.com/ubuntu bionic-updates/universe DEP-11 64x64 Icons [319 kB]
Get:24 http://us.archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 DEP-11 Metadata [2,464 B]
Get:25 http://us.archive.ubuntu.com/ubuntu bionic-backports/universe amd64 DEP-11 Metadata [5,812 B]
Fetched 2,206 kB in 3s (636 kB/s)
Reading package lists... Done
paul@paul-ThinkPad-T420s:~$ sudo dpkg --audit
paul@paul-ThinkPad-T420s:~$ sudo dpkg --configure -a
paul@paul-ThinkPad-T420s:~$

Ok, the package management system is in a consistent status and there is no error shown in the output.

What output do you now receive for the command

sudo apt dist-upgrade

paul@paul-ThinkPad-T420s:~$ sudo apt dist-upgrade
[sudo] password for paul:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages will be upgraded:
  python3-update-manager unattended-upgrades update-manager
  update-manager-core
4 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 632 kB of archives.
After this operation, 1,024 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 python3-update-manager all 1:18.04.11.7 [35.0 kB]
Get:2 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 update-manager-core all 1:18.04.11.7 [8,484 B]
Get:3 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 update-manager all 1:18.04.11.7 [551 kB]
Get:4 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 unattended-upgrades all 1.1ubuntu1.18.04.6 [37.7 kB]
Fetched 632 kB in 1s (519 kB/s)
Preconfiguring packages ...
(Reading database ... 245219 files and directories currently installed.)
Preparing to unpack .../python3-update-manager_1%3a18.04.11.7_all.deb ...
Unpacking python3-update-manager (1:18.04.11.7) over (1:18.04.11.6) ...
Preparing to unpack .../update-manager-core_1%3a18.04.11.7_all.deb ...
Unpacking update-manager-core (1:18.04.11.7) over (1:18.04.11.6) ...
Preparing to unpack .../update-manager_1%3a18.04.11.7_all.deb ...
Unpacking update-manager (1:18.04.11.7) over (1:18.04.11.6) ...
Preparing to unpack .../unattended-upgrades_1.1ubuntu1.18.04.6_all.deb ...
Unpacking unattended-upgrades (1.1ubuntu1.18.04.6) over (1.1ubuntu1.18.04.5) ...
Processing triggers for gconf2 (3.2.6-4ubuntu1) ...
Processing triggers for mime-support (3.60ubuntu1) ...
Processing triggers for ureadahead (0.100.0-20) ...
ureadahead will be reprofiled on next reboot
Processing triggers for desktop-file-utils (0.23-1ubuntu3.18.04.2) ...
Processing triggers for libglib2.0-0:amd64 (2.56.3-0ubuntu0.18.04.1) ...
Setting up python3-update-manager (1:18.04.11.7) ...
Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for systemd (237-3ubuntu10.9) ...
Setting up unattended-upgrades (1.1ubuntu1.18.04.6) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for gnome-menus (3.13.3-11ubuntu1.1) ...
Setting up update-manager-core (1:18.04.11.7) ...
Setting up update-manager (1:18.04.11.7) ...
paul@paul-ThinkPad-T420s:~$

After running this command, the problem still persists

The command line version of the package management system does not show anything wrong.

Please tell, when exactly you see that strange message - at start of Software Center, or after an attempt to install an additional program, etc.
Eventually it would help if you can upload a screen shot to any public internet image hosting service and provide the link to that image.

I select a program to install in the Software center. I click on the Install button. It progresses to 15% and then gives the error message. Then it stalls.
Here are links to 4 screenshots : Let me know if you cannot see them.

https://mega.nz/#!JZMkFA6S!-wtOOVLofi__j4ZYgLIV415DwBuZ3JTCtvPmA0Xclbs

https://mega.nz/#!AQdwSS6A!AdZHrH3cME1Z_d87YHbKGqoLpDqYxJiWd6DxWHpCVh8

https://mega.nz/#!dNEGXaYQ!v9D03s_DBCA56hqF0FrPnfpx64QI0vmw_NZLCe74B0c

https://mega.nz/#!oIdQ1QQZ!DvI6053edi5j9TfdITbt9QK8pZOxiUltaxjFSu7uxbQ

The programs that create problems seem to be all coming from the snap store.

What is the output of the commands

snap version
snap list
snap changes
snap services

(Just a remark: Are you sure that the packages you selected are really those that you want)

I tried similar actions on my system, and did not receive an error message.
Have you tried scrolling in that strange message box, or making it bigger?

Thank you for your help with this. The fact is that the error doesn't appear on another computer, similar to what you indicate that it doesn't show up for you.

Here is the output you request:
paul@paul-ThinkPad-T420s:~$ snap version
snap 2.36.1
snapd 2.36.1
series 16
ubuntu 18.04
kernel 4.15.0-39-generic
paul@paul-ThinkPad-T420s:~$ snap list
Name Version Rev Tracking Publisher Notes
canonical-livepatch 8.0.2 41 stable canonical✓ -
chromium 70.0.3538.110 516 stable canonical✓ -
core 16-2.36.1 5897 stable canonical✓ core
core18 18 442 stable canonical✓ base
gtk-common-themes 0.1-4-g88bc1b2 818 stable canonical✓ -
keepassxc 2.3.4 49 stable keepassxreboot -
skype 8.22.0.2 33 stable skype✓ classic
telegram-sergiusens 1.2.6 68 stable sergiusens -
vlc 3.0.3-1-3-gf09fd0d 365 stable videolan✓ -
wavebox 4.4.0 135 stable wavebox✓ -
paul@paul-ThinkPad-T420s:~$ snap changes
ID Status Spawn Ready Summary
465 Error yesterday at 21:14 -05 yesterday at 21:14 -05 Auto-refresh snaps "skype", "vlc", "canonical-livepatch"
466 Error today at 08:40 -05 today at 08:40 -05 Auto-refresh snaps "skype", "vlc", "canonical-livepatch"
467 Error today at 09:00 -05 today at 09:00 -05 Install "nextcloud" snap
468 Error today at 09:03 -05 today at 09:03 -05 Install "nextcloud" snap
469 Error today at 10:40 -05 today at 10:40 -05 Install "nextcloud" snap
470 Error today at 10:44 -05 today at 10:44 -05 Install "snap-store-proxy" snap
471 Error today at 10:46 -05 today at 10:46 -05 Install "tor" snap
472 Error today at 10:49 -05 today at 10:49 -05 Install "software-boutique" snap

paul@paul-ThinkPad-T420s:~$ snap services
Service Startup Current
canonical-livepatch.canonical-livepatchd enabled active
paul@paul-ThinkPad-T420s:~$

Right. It seems related to the Snap Store. I see that most programs are now Snap. But I found a gnome program/application and it installed fine. And as I said above, on a different computer that error does not show up

I am unable to expand or scroll down the error message.

I am intrigued by your question of whether I want those programs. What should I know about them?

My remark about those programs:
If you look at the page for tor client in the software center, you can see the comment " If you would just like to browse the web with Tor, see Tor Browser instead."
Similar you should be sure whether you want nextcloud or nextcloud-client, etc.
Don't misunderstand me. I do not say that your choice is wrong, but you should be sure which package you want.

Back to your problem:
Which output do you receive for the commands

snap install nextcloud
snap install tor
snap install snap-store-proxy

I installed Tor Browser from the Software Center, and since it isn't SNAP, it installed fine.

I tried installing nextcloud-clien from the Software Center. It is Snap, and it gave the same error.

Then I wen to the termininal and ran snap install nextcloud-client, and it installed fine. Here is the output:
paul@paul-ThinkPad-T420s:~$ snap install nextcloud-client
nextcloud-client 2.3.3+gitab40efe from Nextcloud✓ installed
paul@paul-ThinkPad-T420s:~$

I didn't try installing the other two, as I assumed they would install fine from the Terminal, just like nextcloud-client did

I installed Nextcloud from the terminal: Here is the output.
paul@paul-ThinkPad-T420s:~$ snap install nextcloud
nextcloud 13.0.7snap2 from Nextcloud✓ installed
paul@paul-ThinkPad-T420s:~$

Then I uninstalled it from the Software Center, and it said it uninstalled it.

Then I installed it from the Software Center and it didn't give me an error, indicated it had installed it, but the following message came out on the terminal when I attempted to launch it from the Software Center:
chmod: changing permissions of '/tmp/pids': Operation not permitted
chmod: changing permissions of '/var/snap/nextcloud/9868/apache/logs': Operation not permitted
Making sure nextcloud is setup...
chmod: changing permissions of '/tmp/pids': Operation not permitted
chmod: changing permissions of '/tmp/sockets': Operation not permitted
chmod: changing permissions of '/tmp/pids': Operation not permitted
chmod: changing permissions of '/tmp/sockets': Operation not permitted
chmod: changing permissions of '/tmp/pids': Operation not permitted
chmod: changing permissions of '/tmp/sockets': Operation not permitted
chmod: changing permissions of '/var/snap/nextcloud/common/nextcloud': Operation not permitted
mkdir: cannot create directory '/var/snap/nextcloud/common/nextcloud': Permission denied
chmod: cannot access '/var/snap/nextcloud/common/nextcloud/tmp': Permission denied
chmod: changing permissions of '/var/snap/nextcloud/9868/nextcloud/extra-apps': Operation not permitted
Waiting for PHP...

And there it stalled.

Then I attempted to uninstall it from the Software Center and it indicated it uninstalled it.

 I tried installing Opera from the Software Center (Snap), and it gave the same error message.
I install it from the Terrminal. while installing it indicates it is downloading snap opera(16) from stable "channel"
Here is the final output:
paul@paul-ThinkPad-T420s:~$ snap install opera
opera 57.0.3098.76 from Opera (opera-software✓) installed
paul@paul-ThinkPad-T420s:~$

Then I removed Opera from the Software Center, and reinstalled it from the Software Center, and it worked. So once I install a snap program from the Terminal, then I can uninstall it and reinstall it from the Software Center. But the initial attempt to install a snap program from the Software Center give and error

Sorry, no idea what to do.

What would be the Terminal commands to do a thorough uninstall of Software Center and of the respective cacje, and then reinstall the software center? If it makes sense to try that, what would be the commands to do that?

There is a handful of different packages.

What is the output of the command

dpkg -l | grep -e '-software'

paul@paul-ThinkPad-T420s:~$ dpkg -l | grep -e '-software'
ii gnome-software 3.28.1-0ubuntu4.18.04.4 amd64 Software Center for GNOME
ii gnome-software-common 3.28.1-0ubuntu4.18.04.4 all Software Center for GNOME (common files)
ii gnome-software-plugin-snap 3.28.1-0ubuntu4.18.04.4 amd64 Snap support for GNOME Software
ii python3-software-properties 0.96.24.32.5 all manage the repositories that you install software from
paul@paul-ThinkPad-T420s:~$

This presenttion is easier to read:

paul@paul-ThinkPad-T420s:~$ dpkg -l | grep -e '-software'
ii gnome-software 3.28.1-0ubuntu4.18.04.4 amd64 Software Center for GNOME
ii gnome-software-common 3.28.1-0ubuntu4.18.04.4 all Software Center for GNOME (common files)
ii gnome-software-plugin-snap 3.28.1-0ubuntu4.18.04.4 amd64 Snap support for GNOME Software
ii python3-software-properties 0.96.24.32.5 all manage the repositories that you install software from
paul@paul-ThinkPad-T420s:~$

The following commands should do everything needed to remove, clear and reinstall software center:

sudo apt purge gnome-software gnome-software-common gnome-software-plugin-snap

(Note down the name of all packages that are removed; probably software-center and ubuntu-desktop will be uninstalled as well. This is ok, but you should re-install these packages afterwards as well.)

rm -rf ~/.cache/software-center
rm -rf ~/.config/software-center
rm -rf ~/.local/share/gnome-software

sudo apt install gnome-software gnome-software-common gnome-software-plugin-snap

Finally reinstall the additional packages that have been removed in the first step, probably something like

sudo apt install software-center ubuntu-desktop

Thank you for being with me in this. Following is the output of the above.
paul@paul-ThinkPad-T420s:~$ sudo apt purge gnome-software gnome-software-common gnome-software-
[sudo] password for paul:
Sorry, try again.
[sudo] password for paul:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  python3-netifaces
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  gnome-software* gnome-software-common* gnome-software-plugin-snap*
0 upgraded, 0 newly installed, 3 to remove and 4 not upgraded.
After this operation, 5,626 kB disk space will be freed.
(Reading database ... 245177 files and directories currently installed.)
Removing gnome-software-plugin-snap (3.28.1-0ubuntu4.18.04.4) ...
Removing gnome-software (3.28.1-0ubuntu4.18.04.4) ...
Removing gnome-software-common (3.28.1-0ubuntu4.18.04.4) ...
Processing triggers for mime-support (3.60ubuntu1) ...
Processing triggers for desktop-file-utils (0.23-1ubuntu3.18.04.2) ...
Processing triggers for libglib2.0-0:amd64 (2.56.3-0ubuntu0.18.04.1) ...
Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for gnome-menus (3.13.3-11ubuntu1.1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
(Reading database ... 245076 files and directories currently installed.)
Purging configuration files for gnome-software (3.28.1-0ubuntu4.18.04.4) …
paul@paul-ThinkPad-T420s:~$ sudo apt install gnome-software gnome-software-common gnome-software-plugin-snap
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  apt-config-icons-hidpi gnome-software-plugin-flatpak
  gnome-software-plugin-limba
The following NEW packages will be installed:
  gnome-software gnome-software-common gnome-software-plugin-snap
0 upgraded, 3 newly installed, 0 to remove and 4 not upgraded.
Need to get 2,999 kB of archives.
After this operation, 5,626 kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 gnome-software-common all 3.28.1-0ubuntu4.18.04.4 [2,482 kB]
Get:2 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 gnome-software amd64 3.28.1-0ubuntu4.18.04.4 [489 kB]
Get:3 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 gnome-software-plugin-snap amd64 3.28.1-0ubuntu4.18.04.4 [28.3 kB]
Fetched 2,999 kB in 3s (983 kB/s)
Selecting previously unselected package gnome-software-common.
(Reading database ... 245067 files and directories currently installed.)
Preparing to unpack .../gnome-software-common_3.28.1-0ubuntu4.18.04.4_all.deb ...
Unpacking gnome-software-common (3.28.1-0ubuntu4.18.04.4) ...
Selecting previously unselected package gnome-software.
Preparing to unpack .../gnome-software_3.28.1-0ubuntu4.18.04.4_amd64.deb ...
Unpacking gnome-software (3.28.1-0ubuntu4.18.04.4) ...
Selecting previously unselected package gnome-software-plugin-snap.
Preparing to unpack .../gnome-software-plugin-snap_3.28.1-0ubuntu4.18.04.4_amd64.deb ...
Unpacking gnome-software-plugin-snap (3.28.1-0ubuntu4.18.04.4) ...
Setting up gnome-software-common (3.28.1-0ubuntu4.18.04.4) ...
Processing triggers for mime-support (3.60ubuntu1) ...
Processing triggers for desktop-file-utils (0.23-1ubuntu3.18.04.2) ...
Processing triggers for libglib2.0-0:amd64 (2.56.3-0ubuntu0.18.04.1) ...
Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Setting up gnome-software (3.28.1-0ubuntu4.18.04.4) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for gnome-menus (3.13.3-11ubuntu1.1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Setting up gnome-software-plugin-snap (3.28.1-0ubuntu4.18.04.4) ...
paul@paul-ThinkPad-T420s:~$ sudo apt install software-center ubuntu-desktop
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package software-center is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'software-center' has no installation candidate
paul@paul-ThinkPad-T420s:~$

I don't know what to make of "Package software-center is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'software-center' has no installation candidate"

When I returned to the desktop, it was as if the application was still there. I clicked on it, it opened and said it had to close and reopen, which I did. The error was still there.

But in any event--- I see that I can install snap packages with the terminal using "snap install [package]", so I guess I can it is a useable workaround for me.

Sorry. Ignore my remark about software-center. It was wrong, because it was valid only for Ubuntu 16.04 and older.

Did you remove the cache and config files with the rm ... commands?

What is the output of the command

find ~ -name "*software*" -ls

Yes, I did run

rm -rf ~/.cache/software-center
rm -rf ~/.config/software-center
rm -rf ~/.local/share/gnome-software

This is the output of the command "find ~ -name "*software*" -ls"

paul@paul-ThinkPad-T420s:~$ find ~ -name "*software*" -ls
 11799110 4 drwxr-xr-x 2 paul paul 4096 Dec 6 17:24 /home/paul/.local/share/gnome-software
  1704647 8 -rw------- 1 paul paul 7132 Dec 31 1999 /home/paul/.local/share/torbrowser/tbb/x86_64/tor-browser_en-US/Browser/TorBrowser/Data<email address hidden>
  3150272 4 -rw-rw-r-- 1 paul paul 2445 Dec 5 06:13 /home/paul/snap/wavebox/138/.local/share/mime/x-content/unix-software.xml
  3150060 4 -rw-rw-r-- 1 paul paul 2563 Dec 5 06:13 /home/paul/snap/wavebox/138/.local/share/mime/x-content/win32-software.xml
  3149904 4 -rw-rw-r-- 1 paul paul 2468 Dec 5 06:13 /home/paul/snap/wavebox/138/.local/share/mime/x-content/software.xml
  4199362 4 -rw-rw-r-- 1 paul paul 2445 Oct 18 19:34 /home/paul/snap/wavebox/135/.local/share/mime/x-content/unix-software.xml
  4199160 4 -rw-rw-r-- 1 paul paul 2563 Oct 18 19:34 /home/paul/snap/wavebox/135/.local/share/mime/x-content/win32-software.xml
  4199004 4 -rw-rw-r-- 1 paul paul 2468 Oct 18 19:34 /home/paul/snap/wavebox/135/.local/share/mime/x-content/software.xml
  4197451 4 -rw-rw-r-- 1 paul paul 2445 Dec 5 06:13 /home/paul/snap/wavebox/139/.local/share/mime/x-content/unix-software.xml
  4197443 4 -rw-rw-r-- 1 paul paul 2563 Dec 5 06:13 /home/paul/snap/wavebox/139/.local/share/mime/x-content/win32-software.xml
  4197438 4 -rw-rw-r-- 1 paul paul 2468 Dec 5 06:13 /home/paul/snap/wavebox/139/.local/share/mime/x-content/software.xml
  4196644 4 -rw-rw-r-- 1 paul paul 2445 May 21 2018 /home/paul/snap/vlc/190/.local/share/mime/x-content/unix-software.xml
  4196428 4 -rw-rw-r-- 1 paul paul 2563 May 21 2018 /home/paul/snap/vlc/190/.local/share/mime/x-content/win32-software.xml
  4196256 4 -rw-rw-r-- 1 paul paul 2468 May 21 2018 /home/paul/snap/vlc/190/.local/share/mime/x-content/software.xml
  1835732 4 -rw-rw-r-- 1 paul paul 2445 Jun 4 2018 /home/paul/snap/vlc/365/.local/share/mime/x-content/unix-software.xml
  1835518 4 -rw-rw-r-- 1 paul paul 2563 Jun 4 2018 /home/paul/snap/vlc/365/.local/share/mime/x-content/win32-software.xml
  1835347 4 -rw-rw-r-- 1 paul paul 2468 Jun 4 2018 /home/paul/snap/vlc/365/.local/share/mime/x-content/software.xml
  4588201 4 -rw-rw-r-- 1 paul paul 2545 Aug 31 08:15 /home/paul/snap/keepassxc/40/.local/share/mime/x-content/unix-software.xml
  4588054 4 -rw-rw-r-- 1 paul paul 2667 Aug 31 08:15 /home/paul/snap/keepassxc/40/.local/share/mime/x-content/win32-software.xml
  4588013 4 -rw-rw-r-- 1 paul paul 2556 Aug 31 08:15 /home/paul/snap/keepassxc/40/.local/share/mime/x-content/software.xml
  4981674 4 -rw-rw-r-- 1 paul paul 2545 May 10 2018 /home/paul/snap/keepassxc/39/.local/share/mime/x-content/unix-software.xml
  4981668 4 -rw-rw-r-- 1 paul paul 2667 May 10 2018 /home/paul/snap/keepassxc/39/.local/share/mime/x-content/win32-software.xml
  4981664 4 -rw-rw-r-- 1 paul paul 2556 May 10 2018 /home/paul/snap/keepassxc/39/.local/share/mime/x-content/software.xml
  4721034 4 -rw-rw-r-- 1 paul paul 2545 Sep 7 07:15 /home/paul/snap/keepassxc/49/.local/share/mime/x-content/unix-software.xml
  4720816 4 -rw-rw-r-- 1 paul paul 2667 Sep 7 07:15 /home/paul/snap/keepassxc/49/.local/share/mime/x-content/win32-software.xml
  4720646 4 -rw-rw-r-- 1 paul paul 2556 Sep 7 07:15 /home/paul/snap/keepassxc/49/.local/share/mime/x-content/software.xml
  2231172 12 -rw------- 1 paul paul 9216 Jun 24 2017 /home/paul/Documentos\ CEAS/Internet/Linux/ubuntu-software\ app.doc
  2231061 48 -rw------- 1 paul paul 48743 Feb 3 2014 /home/paul/Documentos\ CEAS/Internet/Linux/Other\ software\ respositories.png
  1705808 4 -rw-rw-r-- 1 paul paul 83 Dec 7 08:50 /home/paul/Documents/Transit/.~lock.Terminal\ printout\ of\ software\ center\ uninstall-reinstall.doc#
  1705809 20 -rw-rw-r-- 1 paul paul 19968 Dec 6 17:30 /home/paul/Documents/Transit/Terminal\ printout\ of\ software\ center\ uninstall-reinstall.doc
  1575455 48 -rw------- 1 paul paul 48743 Feb 3 2014 /home/paul/Documents/Computer\ Info/Linux/Other\ software\ respositories.png
 11931391 4 drwxr-xr-x 9 paul paul 4096 Jun 4 2018 /home/paul/.cache/gnome-software
   131541 4 -rw-rw-r-- 1 paul paul 2716 Dec 1 11:25 /home/paul/.cache/gnome-software/icons/e50c2314342cfdf963d1b3c50e82ea15cb42cc37-software-boutique.png
paul@paul-ThinkPad-T420s:~$

Ok. The only file/directory requiring deletion is ~/.cache/gnome-software

It seems that I should have added

rm -rf ~/.cache/gnome-software

Hmmm. I ran rm -rf ~/.cache/gnome-software, but the error persists.

After removing the cache you have to restart the program.

killall gnome-software
rm -rf ~/.cache/gnome-software
gnome-software

I did what you suggested above. Then I attempted to install Opera, a snap package. This is the output, in case it offers a clue to what is happening:
12:09:36:0400 Gs failed to call gs_plugin_app_install on snap: cannot perform the following tasks:
- Download snap "opera" (17) from channel "stable" (invalid credentials)

Are you using a web proxy that requires authentication?

As far as I know I am not using a proxy. Is there a specific way to verify that?

Note that when I install snap from the Termianal, it installs fine

Note that when I install snap from the Termianal, it installs fine

"Note that when I install snap from the Terminal, it installs fine" - I know. The strange thing is that it doesn't from within gnome-software.

Is there any output on one of the following commands:

sudo grep -R roxy /etc/apt/*
grep roxy /etc/environment
grep roxy /etc/bash.bashrc
grep roxy ~/.bashrc
echo $http_proxy
echo $ftp_proxy

No output:
paul@paul-ThinkPad-T420s:~$ sudo grep -R roxy /etc/apt/*
[sudo] password for paul:
paul@paul-ThinkPad-T420s:~$ grep roxy /etc/environment
paul@paul-ThinkPad-T420s:~$ grep roxy /etc/bash.bashrc
paul@paul-ThinkPad-T420s:~$ grep roxy ~/.bashrc
paul@paul-ThinkPad-T420s:~$ echo $http_proxy

paul@paul-ThinkPad-T420s:~$ echo $ftp_proxy

paul@paul-ThinkPad-T420s:~$

Sorry, I am running out of ideas.

I am very grateful to you for all your orientation and help, for your patient persistence with me in this, and for the teaching this has represented for me. Thank you very much.

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