That video is from 2016 and by now extremely outdated. E.g. /etc/default/plexmediaserver or it’s .prev backup are not used at all. Moreover, I suppose the author tells you to edit this file to change the user to pi. That’s also wrong.
Please only ever use our official setup instructions. They will work Please undo any permission changes or simply just purge your current Plex package and start from scratch: sudo apt-get purge plexmediaserver-installer.
Please undo any permission changes or simply just purge your current Plex package and start from scratch: 