Hey FinnJack,
If this is a new Apple ID, you'll likely need to sign in on the web, accept the terms and conditions and then you can use that Apple ID for TV+ features.
See more info here:
"If you see "No iTunes Store account" on your Apple TV or are unable to complete accepting the terms and conditions, follow the steps below. Then you can download apps, subscribe to streaming services, and make purchases with your Apple ID.
Sign in on the web
- Go to tv.apple.com or music.apple.com.
- Choose Sign In, then sign in with your Apple ID.
- Follow the prompts to complete setup.
If you're using the Apple TV app on a non-Apple device, you can also sign in on the web to set up your Apple ID.
If you're still unable to make a purchase on your Apple TV
Try the steps again, then return to your Apple TV and sign in with your Apple ID and password. Accept the terms and conditions to complete setup."
Cheers!