As discussed over the phone and via email.
The issue looks to be trying to access the BT TV app as opposed to the BT Sport app.
The BT Sport app should work with your account details.
If there are any further problems please drop me a PM.
Thought I would put this here as well as in email so that if anyone else has the same issue they can try this solution.
TVE_A914 is a BT TV app error and requires a BT TV subscription. BT Sport app is a separate app and so should be able to log onto that app all okay.