Cannot upgrade Ubuntu to 20.04 because of Adobe

Solution 1:

As Ray pointed out in a comment, Flash became end of life at the year end. It looks like you should uninstall all the Flash related stuff before upgrading. I'm thinking of these packages in the Ubuntu archive:

adobe-flashplugin
flashplugin-installer
pepperflashplugin-nonfree
browser-plugin-freshplayer-pepperflash

If you have some repository provided by Adobe enabled, you should disable that from your apt sources.

If you installed Flash some other way but through .deb package(s), e.g. directly from the Adobe site, you should know best yourself which files to remove.