One room in a group acts as master. Make sure you add the Arc room to the Era room, not the other way around.
Thank you. I am not sure I understand…

If I go in the ERA 100 section of the app, I can group it with other Sonos devices (other rooms) but I cannot see the Arc Ultra. The app says that only Era 100, Era 100 pro, Era 300, Five , One and One SL can be added to a zone.
If you could describe in a bit more detail what you meant I would be very grateful
If the Arc Ultra is not visible in the app, did you set it up to be part of the system wit( the Era and the One, or as a separate system?