https://support.plex.tv/articles/201638786-plex-media-server-url-commands/ wait wrong one
https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
All the good terminal commands here:
https://www.linuxbabe.com/ubuntu/install-plex-media-server-ubuntu-18-04
systemctl status plexmediaserver
sudo systemctl restart plexmediaserver
Extra useful chat: