Sounds like you need to adjust the permissions of /data/plex/ and its content to be visible to the local plex user.
As for 20.04 LTS – some consider it too early / risky to move on a newly released version and prefer waiting for the “.1” release (first bug fix update).
Personally I’m on 18.04 and will wait at least until they provide that update via apt-get distr-upgrade.
