For reference… the current version of Plex Media Server will run on PMS 16.04 or newer. My guess is you might need to update to a newer version due to some dependencies of Plex before Ubuntu ends their support of the platform 
If you’re new to Linux I can highly recommend the Linux tips from the Tips & Tricks section. Those will be particularly helpful when it comes to correctly mounting a drive / assigning permissions for Plex to pick-up your media.
