I may be wrong but i think you need to fallow the old post first.
before you can use sudo systemctl edit plexmediaserver
also i have changed the umask to let the group to have full access to the media.
here is what i use
[Service]
# These values are only needed if you wish to change user & group
User=plex
Group=pi
# This is needed to change the default umask
UMask=002
