Unplayable File Issues
Some files may be unplayable on your Apple TV. This can be due to the way a video was created, creating an incompatible video format or file that can’t be played back by the Apple TV hardware.
If you are creating your video in Final Cut Pro X, you can make sure it is compatible by exporting for Apple Devices, either through the Share menu or by sending to Compressor.
For iMovie, when exporting, choose Quality “High” not “Best (ProRes)” to enable playback on Apple TV.
Another option is to use the QuickTime Player to export it into a more compatible format:
With the video open in QuickTime Player, select Export As 1080p (if you don’t have a 4K AppleTV)
Select Greater Compatibility (H.264) for the type
Then just swap the new version for the old.
More information is available from Apple’s support pages.