Ubuntu 20.04.3 lts or ubuntu 21.10 for plex server

Please forgive my candor. I speak to your best interests.

Do not use Linux as a Plex server until you have SOME mastery of Linux.

The desktop GUI will not help you.
You need to use the command line for all system-level tasks.

Stay with Windows or MacOS.
Learn in a VM.
Switch AFTER you have mastered the required skills.

Some of the required skills are shown here: