It takes two to tango. The 'losing' carrier must give up the number, and the 'winning' carrier must take it. If either one is incapable, it cannot happen. If they are merely reluctant or incompetent, remedies exist, it just takes longer.
It sounds like in your case, T-Mobile is the 'winning' carrier, and they cannot take that number.
When porting a number, the 'winning' carrier must be able to receive/service that number in the rate center to which it belongs.
In other words, if you are trying to port a Kalamazoo number, the 'winning' carrier must be able to handle the Kalamazoo prefix you are trying to port.
Try a different carrier. Maybe a TracPhone or AT&T GoPhone (is that still the current brand for AT&T prepaid?) or Virgin Mobile.