Check out the discussion on ask ubuntu, they explain the issue and how you can work around it. It's because PGAdmin4 lacks support for Python 3.8, as you can see on Pypi.
You'll either need to wait until a compatible release for Ubuntu 20.04 and Python 3.8 is published or work around it by installing PGAdmin in a Python 3.7 environment via something like virtualenv, conda or Docker.
