The other commands need a container name, which I don't have. I posted log earlier. Unless you mean a different log?
The service/container name is "docker.linuxserver.plex"
"systemctl status docker.linuxserver.plex.service" will tell you the current status of the systemd service that is responsible for creating and starting the container
"journalctl -u docker.linuxserver.plex.service" will show you the log
Since you mentioned there is no running container on your system when the addon is enabled, there must have been an error with systemd. We need to see the log to troubleshoot that.
I just installed the addon on a fresh test instance and it fired right up