@keith8
Welcome to the forum. You can only have four devices registered to your account for watching NOW TV on at any one time. You will need to remove one of your current four boxes from your Devices page if you want to use your stick to watch NOW TV on.
https://account.nowtv.com/device-management
But you will still be able to use all the other apps on the box you remove, it's only watching the NOW TV app you won't be able to do.
NOW are looking for users to join their Customer Feedback Group and help improve the service. If you're interested read more here and sign up.