It seems Avogadro was phased out in Ubuntu 20.04 (Focal Fossa) and derivatives like Mint 20 (Ulyana). Now the version available in the repositories is Avogadro 2. If you had Avogadro installed in 18.04, and did a upgrade, it is replaced with Avogadro 2.
I faced a minor issue while trying to adapt my workflow to the new version. While drawing some molecules, I found the new location of the geometry optimization tool, under Extensions > Open Babel > Optimize geometry. The problem was, the relevant entry, together with others in the same location, were grayed out, and unselectable, as the image below shows:
It was a headscratcher, and I spent some time figuring out what was the problem, and opened this question afterwards to document the solution, to help other people that may face the same issue.