How to install Cubic in Linux Mint 19.x ?

[1]
Your sources should look something like this:

    deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
    deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
    deb http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
    deb http://security.ubuntu.com/ubuntu bionic-security universe

[3] Add the cubic-wizard repository key...

    $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B7579F80E494ED3406A59DF9081525E2B4F1283B

[4] Verify the key was added. You will see the following line...

    pub rsa4096 2015-11-05 [SC]
          B757 9F80 E494 ED34 06A5 9DF9 0815 25E2 B4F1 283B
    uid [ unknown] Launchpad PPA for PJ Singh

[5]
Update your packages cache:

    $ sudo apt update

[6]
Optionally, I suggest upgrading all your packages to the latest available versions.

    $ sudo apt upgrade

[7]
To to circumvent the base-files dependency issue, we can temporarily install a "fake" qemu-system-gui package.

    $ sudo apt-get install equivs
    $ cd ~/Documents
    $ equivs-control qemu-system-gui
    $ sed -i "s|^Package.*|Package: qemu-system-gui|g" qemu-system-gui
    $ sed -i "s|^# Version.*|Version: 1:4.2|g" qemu-system-gui
    $ equivs-build qemu-system-gui
    $ sudo dpkg -i qemu-system-gui_4.2_all.deb

[8]
Install Cubic

    $ sudo apt install --no-install-recommends cubic

[9]
Finally, uninstall the "fake" qemu-system-gui package, while keeping Cubic.

    $ sudo dpkg -r --force depends qemu-system-gui

Bug #1952680, "Cannot install Cubic in Linux Mint 19.x" has been opened to address the installation issue.

Fix released in version 2021.11-63.

To install Cubic...

    $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B7579F80E494ED3406A59DF9081525E2B4F1283B
    $ sudo apt-add-repository universe
    $ sudo apt-add-repository ppa:cubic-wizard/release
    $ sudo apt update
    $ sudo apt install --no-install-recommends cubic