You may want to test your ignition switch. When they fail, they'll do what you're describing. If it were just the Passlock acting up it should crank but not start or start for a second and stall.

The ignition switch wiring connector looks like this. The White with black stripe wire isn't labeled but it's the battery voltage (12 volts + or -) that's always hot from the 10 amp fuse in your underhood fuse box.

The wiring diagram shows the details.

I found a video of Eric O testing the ignition switch wiring on a Colorado and I like the video because he shows you the wiring diagram and explains how the circuits work. He shows how to access the ignition switch and how to backprobe the connector with a pin and what the individual wire voltages should be in the different key positions. After the repair he shows how to perform the security relearn procedure so the BCM accepts the new ignition switch.

He's using a Power Probe for the voltage tests. You can use a $10 multimeter. 

The video is from 2019 and he says that you can't by the OEM ignition switch without buying the ignition lock housing with it. It's 3 years later, maybe you can now. Look into that.

Just be sure to buy OEM (AC Delco) because your BCM is using the switch for voltage signals.

Here's what it looks like (the switch is the black part):

 He's a professional mechanic. If that's the way it comes from GM then that's the way he installs it.

He gets paid by the hour.

If I were doing this job for my own vehicle and even if I had to buy the ignition lock housing to get the new switch I wouldn't install the entire housing because you have to remove the airbag and the steering wheel and it's too much unnecessary work.

I'd remove the ignition switch from the new housing and screw it onto my existing housing.

Here's the video:

https://www.youtube.com/watch?v=JXkXuz1yuf8

Do you have another good key to try?

A thorough read of your owner's manual section on security features might give you the answer.