Something is causing the app to be unable to contact the remote servers.
See if this document has clues that apply to your situation: If you can't watch or stream movies and TV shows in the Apple TV app - Apple Support
“Video not available” may mean no valid reaction from the remote server.
- Verify that your router doesn’t block Apple’s servers (or its CDN) through a firewall setting.
- Verify that your router doesn’t block ports that Apple services like to use.
- Verify that you use a good DNS on your router, like the one from your ISP, which gets updated with all server names that Apple might use.
- Verify that your Roku has the correct date and time and timezone, as Apple may be more strict about that (TLS) than other content providers.
Else
- Check for Roku OS software updates.
- Check for Apple TV app updates, if not set to do so automatically.
- Try resetting the Apple TV app.
- Try deleting and re-installing the Apple TV app.