Problem: `apt` can't find packages


apt did not find a package which should be there. This can occur after adding a new repository but also suddenly.


Update the whole apt cache from scratch.

apt clean && apt update

This normally solves the problem or at least gives a hint whats wrong with the configuration.

You can check for a newly added repository by

apt policy | grep <repo-id>

or for a specific package via

apt policy <package>