Sounds like you are running into conflicts for some reason. The root cause might be that you enabled non-Ubuntu package repositories after upgrading that aren't 100% compatible with Ubuntu 20.04? If so consider removing them from /etc/apt/sources.list.d/ and run apt update.
If that doesn't help, I would recommend running aptitude interactively. It proposes solutions and allows you to resolve them in a semi-automatic way. You might also want to keep an eye on "held" packages. It often helps to upgrade them. To do so start aptitude in interactive mode and press Shift+U, then G twice.