Ah. I see. Also this guide is wrong and editing /etc/default/plexmediaserver.prevdoes nothing 
Did you change the owner of /var/lib/plexmediaserver to osmc?
Please show me the permissions of this directory: ls -ahl /var/lib/plexmediaserver/
Please restore the ownership of /var/lib/plexmediaserver to plex:plex sudo chown -R plex:plex /var/lib/plexmediaserver.
You might want to add user plex to the osmc group: sudo gpasswd -a plex osmc.
FInally, restart the PMS service one last time.
And for the sake of completion: Here are the official setup instructions:
