Thecus is currently not supported due to their lack of updates to their host OS. The current Plex version requires a few newer libraries than what Thecus OS currently has. There is another thread that covers this- it’s about 6-9 months old. Thecus has not responded to multiple requests to update their OS to include the newer libraries.
Here’s the older request: Plex server on Thecus N5550 show an update is available unable to download
