I enquired about this issue in Aug 2020 (see https://askubuntu.com/q/1271611/541417). @SteveLanders solution worked for me. https://askubuntu.com/a/1312345/541417.
The following reinstallation procedure that he had shared worked for me: sudo apt install --reinstall python3-six python3-certifi python3-requests python3-idna sudo apt install --reinstall software-properties-gtk software-properties-gtk -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1830914 Title: software-properties-gtk does not run at all! To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/1830914/+subscriptions -- ubuntu-bugs mailing list firstname.lastname@example.org https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs