So for the past few weeks, I have been unable to sign in on my Apple TV. It is listed as a compatible device. what-set-top-boxes-streaming-devices-can-i-use-to-watch, I've just raised it on live chat help, apparently Now are aware of the issue affecting Apple TV and are looking to resolve.
Thankfully I’ve not been kicked out on my Apple TV box, so no issues with me.
Take it the system is fully updated etc?
Have you done a delete and reinstall of the app?
I just tried signing out of Now on my Apple TV 4K, and signing in again, and it worked fine for me.
So I guess the issue isn’t universal. Or maybe I should have powered down and powered back up during the process, or some such specific action?
I’m in the same boat as well, recently signed up to NOW tv and it works on my iPhone and iPad but not my Apple TV 4K. No idea why. I get the emails saying “new device […] did you just sign in” but not actually getting signed in.