tv movie on Apple TV+

Unforgotten needs to be purchased first before it can be watched. It is not included with the Apple TV+ subscription as its not Apple Original content.

Is this happening on your Mac, or on a different device?

1. Some devices (Fire TV/Android TV/Google TV) can show Store items, but do not support on-device transactions through Apple at this time. Then buy/​rent/​subscribe on a different device instead, where it will then be authorized for other devices as well, through the matching Apple ID account.
2. Some titles may have a future release date. Then those are not available to play yet.

Which unplayable title did you add to Up Next?

I get this:

If you are using the Apple TV+ app on a 3rd party TV or device, you won't be able to watch it, as its not part of the Apple TV+ subscription service, and requires it be bought or rented. Abilities not available in the Apple TV+ app on 3rd party devices.

Unforgotten is not AppleTV+ content