[How to] Fix the 'Repository is not signed' error when you already have imported a signing key for that repository

This just happened to me, but there is a chance that you'll encounter the 'Repository is not signed' and you have already added the public signing key for that repository when you install an app. It's a pretty simple fix:

  1. Go to Software & Updates (via the software updater) then go into the authentication tab.

  2. Click 'Import Key File...'

  3. In /usr/share/keyrings select the gpg key that signs the repository you were having issues with. It should include the repository's name.

If you follow these steps, the issue should not appear again.