I am sorry that you have not been helped yet. I just came online and saw your question.
Check to see if the brake lights are working. the brake switch is part of the interlock system and if not working can cause this.
If ok, try the 30 minute relearn procedure
30 minute GM relearn procedure:
1-Turn ON the ignition, with the engine
2-Attempt to start the engine, then release the key to ON (vehicle will not start).
3-Observe the SECURITY telltale(this is the security light), after approximately 10 minutes the telltale will turn OFF.
4-Turn OFF the ignition, and wait 5 seconds .
5-Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes (the vehicle is now ready to relearn the Passlock(TM) Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK). IMPORTANT: The vehicle learns the Passlock(TM) Sensor Data Code and/or password ***** the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6-Start the engine (the vehicle has now learned the Passlock(TM) Sensor Data Code and/or password).
If still no go, then this is likely a fault ignition switch itself and will need to be replaced
Please remember to rate service using the rating page so that I may receive credit. Thanks