I'm having the same issue... apt.postgresql.org is in the "sources.d", I can see it is "Hit" by "apt update", but still no pgadmin4 installation candidate found.
It looks like focal (20.04) is simply not currently supported (although multiple articles indicate it was at one point). Earlier versions are all there. Alas, trying to install them leads to the requirement of python3-psyco2 < 2.8, but 2.8 is the focal version. Perhaps a build fail caused it to drop from repos, or an upstream version change is causing a problem... it's beyond me at this point...
Anyway, I'm not up to working that hard for it... so it's our good old friend "psql" for me right now :-)
