Installed numerous times - will not recognise a single file

As divideby0 rightly said, on DSM 7 DON’T put your media files n the PlexMediaShare share.

Again to be sure:

DO NOT PLACE MEDIA ANYWHERE INSIDE THE PlexMediaServer SHARED FOLDER

You can put them in any other share (Plex is fine, but to avoid confusion it better to create a different one), just remember that you need to give the system internal user PlexMediaServer permission to R/W on that share.

For reference.
Instructions:

Assistance for problems: