Plex can't find my library on Terra Master F5-421 NAS. Is it Linux, BTRFS, Security or ? Please Help!

Sounds fun. Be sure you try the simplest of solutions, shutting the NAS off,
then starting it up a couple minutes later.

Check out these articles about Terra Master hardware.
Let us know what you try and what doesn’t work out.
It could be a simple answer that I don’t know, but at least you have
some things to read.

Problems can multiply if you get it partially running then make major changes to it.
You might have to completely uninstall PMS and go again. A description of how to
remove the Plex app and download it again was given here: