Can’t add media to libeary

[tom80H]

Sounds like a permission issue.
habe a look at this post…

Synology NAS can't add folders

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…