On Linux, one user account can’t see another user’s files without expressly granted permission. This is how Plex is implemented on Synology; as a regular user account versus being an all-powerful administrative user.
If I may suggest, while you’re setting this up,
-
Make a shared folder (Control Panel - Shared Folders) freestanding instead of using the Synology defaults. Synology defaults have the habit of erasing previously granted permission.
-
The procedure to give permission to Plex is included here.
