In most cases, the Plex installation and associated files/directories will be owned by the plex:plex user:group. You may need to switch users to access the content for some platforms.
On Debian, the logs should be located in /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Logs
I am looking in “/var/lib/plexmediaserver/Library/Logs/” and they are in “/var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Logs/”.