It seems that the Apple TV 4K is having problems recognizing iTunes hosted on a PC with a media library. Streaming apps seem to work but playing content from iTunes is not working. Even rebooting the Apple TV 4K unit has not been successful.
Apple has also not update iTunes for years suggesting they are losing interest for some reason. At the same time Apple TV has not been updated for H.265 which was disabled and H.266 which is the more recent standard.
Itunes can H,.264 content but H.265 is not playable. H.266 is equally unplayable. It seems that Apple needs to regain focus before consumers abandon them completely.
Plex runs on most NAS units so it is an alternative platform. Plex runs on Apple TV 4K fine and it can stream many channels in addition to playing content from local hosts including Linux, OS X, Windows and many NAS units.
iTunes on the Windows Store is unstable and very prone to crashing. Use the version on Apple’s site for other operating systems which is at least stable.