The Roon Apple TV app is not made by Roon and is not supported by them either. It is a third party app that uses Roon’s extension architecture to function. You can get support here Apple TV Extension - Roon TV Remote
To find the code for your AppleTV airplay, go to the AppleTV settings, look for airplay and find the code under access restrictions or something similar. If it’s not set, make a code. Then when you try and play the the AppleTV zone you can enter this code for Roon to gain access.