would hazard a guess that there is or has been an issue with the data being transmitted to/from the gearbox (guessing it's an auto?)
this may be an issue with gearbox ecu to main engine ecu, or simply that the code reader you used won't communicate with gearbox ecu.
not all readers do everything, some only interrogate main engine ecu, as that is the minimum laid down under obdII
can the code be reset ?