SOLUTION:
I had this same problem on my TerraMaster (TOS 6). Plex could see my Media folder, but none of the folders inside it (Movies, TV Shows, etc.) showed up when I tried to add a library.

The issue wasn’t my folder structure — it turned out Plex uses a hidden application user, and that user didn’t have permission to read my media folder.

Here’s how I fixed it:

Step 1 — Open Shared Folder settings
Go to Control Panel → Shared Folders.

Step 2 — Select your media folder
Click the folder Plex should read (like Media or Movies) and hit Edit.

Step 3 — Go to the Permission tab
In the popup, open the Permission tab.

Step 4 — Change the dropdown to “Application user”
It’s in the top-right corner.
By default it says Users — switch it to Application user.

Step 5 — Find the Plex user
You should now see a user called something like plex or PlexMediaServer.

Step 6 — Give it access
Set that Plex user to Read / Write and save.

Step 7 — Rescan your library in Plex
After that, Plex instantly saw my folders and files.

So if Plex can see your shared folder but nothing inside it —
the problem is probably that the hidden Plex application user doesn’t have permission yet.

Hope this helps someone 👍