Running into errors drives me nuts sometimes and I haven't see this error before. While we weren't notified of any server errors going on earlier this week it's possible the issue has been resolved. Are you still getting this error message? If so, are you BRP (Billing Responsible Party)?
That is an easy fix all you must do is contact T-Mobile Customer Service and they can assign your number to be the primary line on My T-Mobile. Call 1-800-937-8997 to complete this request.