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Are you running Plex Media Server on the Synology or on the Mac?
You tagged the thread server-mac, but from the screenshot it appears PMS is running on the Synology. -
What version of PMS is installed?
Settings → Bill’s NAS → General. -
What version of DSM is installed on the Synology?
Login to the NAS → Control Panel → General -
Where on the NAS are your media files located (what shared folder)?
If running Plex Media Server on the Synology, here’s some documents that can help.