Might help you. After every fresh install I add the following
sudo nano /etc/default/plexmediaserver
Replace the "plex" user account with your own account.
then add lucas user to group:
sudo addgroup plex lucas
sudo addgroup lucas plex
Then to stop and start the service
sudo service plexmediaserver stop
sudo service plexmediaserver start
I add the following in .bash_aliases for easy start/stop
alias start='sudo service plexmediaserver start'
alias stop='sudo service plexmediaserver stop'
If you are trying to Limit what Plex Server can see I would look at Firejail. I use it constantly and I know you can control network connections with it so you may be able to control Plex.
sudo apt install firejail
Home Page: https://firejail.wordpress.com/