My dad is having that problem right now. A rebuilt 08 transmission and he has to download a file from GM and plug it in to tell the computer that it has a transmission. He was fiddling around with his scanner last night to see if the file could be put on it I think. Otherwise he doesn't have much of a way to hook up to the trucks computer.
Guessing the 6L80? The reason is the Trans control module is built into the valve body and requires programming to the vehicle as it could be put in a few different platforms. This one isn't that bad, but you need a Tech-II, MDI, or aftermarket calibration tool to program it. Standard OBD-II scanner can't do it.