How to reset GM anti-theft system

Place key in ignition, turn to ON position. (do not start)
Leave in ON position for EXACTLY 10 minutes.
As SOON as the anti-theft light turns off, then IMMEDIATELY start the car.
Then anti-theft is now reset.

This could be caused by either a faulty Crank Shaft Position Sensor or the anti-theft system. Try put key in, turn to ON position for EXACTLY 10 minutes, As SOON as anti-theft light shuts off or stops flashing, then IMMEDIATELY start the engine.

put in key in ignition, turn to ON position (do not start) leave in on position for EXACTLY 10 minutes. As SOON as anti-theft light turns off, IMMEDIATELY start the car.

You may have a faulty ingition. Otherwise you have the very common GM anti-theft system. If you have a flashing anti-theft light with key in igniton, then this is what you should do:

Put key in ignition, turn to ON position (Do NOT Start)
Leave in on position for EXACTLY 10 Minutes
Then while watching the anti-theft light, as SOON as the light turns off after 10 minutes, START CAR IMMEDIATELY after light turns off.

This will reset the anti-theft

Do you have any check engine lights on? Is your anti-theft light flashing? If so, put key in igniton, turn to on (NOT START) leave in on position for EXACTLY 10 MINUTES
As SOON as theft light turns off, try to start.

There is a problem with your key.