Something sticks in my mind about being unable to update over the top of an existing install if a major release is missing. In your case Kodi 19. I may be wrong.
The simplest solution may to uninstall your current version and do a fresh install of 20 either from Google play or the official Kodi site. Of course you will have to reinstall all your add-ons if you do that (unless you can back up your configuration, though I don't know how successful reinstalling that would be).
You might be able to grab the latest Kodi 19.x version from the official Kodi site and see if that will install over the top. You might then be able to update THAT to 20.
At any rate be aware that developers may not have built Kodi 20 compatible versions of their add-ons yet so many may not work.