What device are you trying to watch on?
Taking part in the Xbox One Update Preview?
The NOW TV app might not work if you’re participating in the Xbox One Update Preview.
You can still watch NOW TV on loads of other devices though – go to Manage devices to control what devices you use to watch NOW TV.
I have no idea what it is, as I do not have this device. But what I quoted was the sum total of NowTV Help for why your box might not work.
To go further, I think you need to get in touch with Live Chat:-
Get in Touch (all the options, and the best place to start):-
On a screen with big buttons all around, you may find the link you need is under a tiny green arrow, so be vigilant.
Live Chat and Send Us a Message:-
On Live Chat, when you can get onto it, keep your answers short, sweet and prompt, or it will time out on you.
If the person you are talking to doesn’t seem able to understand your issue, ask to be escalated to the next level of help.
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Use this link to get to live chat where you don’t deal with the “bot”
Make sure you click “chat online” within the green box
Alternatively the email address is in the picture below