If you hold down the Menu button and the down arrow on the remote for 10 sec until the LED on the front of the Apple TV begins to flash. Let go of the buttons and the Apple TV will do a soft reboot.
This has been happening to me several times a week and doing this seems to fix the issue.
I fixed mine by just going back to the stv menu and trying to play it again.
In my case I was streaming the content from my iPad.
I'm getting this error for several purchased titles at the moment and I'm not sure what the deal is. Is anyone else experiencing this problem?
Apple acknowledged the issue: https://www.apple.com/support/systemstatus/
Edit: Some people have reported this issue on iOS 15.7, and the Mac App Store as well, so this seems like it is not related to the new update.
Edit 2: Looks like Apple has fixed the issue (not reflected on the System Status page). Other users have reported that restarting their devices helped fix the issue