Hi Johnnyess58. Thanks for posting and welcome to our community.
I am really sorry to hear you're having issues with watching BT Sports on your TV Go App. I realise this may sound a silly question. But is BT sports still a part of your subscription? Were you able to access on your box?
Yes, I have the full TV package and can see BT sport on my TV but am still getting a service not available message on the TV Go app.
I have done some searching and have seen something about a technical problem that should have been resolved by 9 November, but it obviously has not been fixed. The general lack of info about this is pretty poor!