https://appletoolbox.com/apple-tv-app-for-roku-not-working-how-to-fix/
"Try uninstalling, restarting, and clearing cache on Roku
The most obvious response to an app not working on just about anything is to uninstall it, and then re-install it. But even before that, you can attempt to knock out the cobwebs by restarting your Roku first. Here’s how you can restart your Roku:
Press the Home Button five (5) times on your remote
Press the Up Arrow once
Hit the Rewind Button twice
Press the Fast Forward button twice
This will automatically restart your Roku after a few moments, and then you can attempt to open the Apple TV app again. When restarting the Roku, this method automatically clears the cache on your Roku, essentially getting two birds with one stone.
If that doesn’t work, you’ll want to uninstall the Apple TV app and then reinstall it onto your Roku device."