Are you Home Sharing? On a 4K AppleTV?
Reauthorize the computer and try again, restarting the Apple TV if necessary.
Failing that sign out of everything on the Apple TV box, sign in again.
Failing that, observe the file type if downloaded and make sure it is not a HLS video package. You said you downloaded and can I guess you Home Share? 4K purchased content does not play via home sharing on the appleTV, or at least I can't get it to play unless I stream it.
Set your prefs in iTunes (TV app) to "best available" uncheck multichannel & HDR and download again to get the 1080 file version.
oh, Remove download first to clear the way.
restart again to be sure. [hold top two buttons on Siri remote til light blinks]