I realize this will seem like a question that's been posted so many times. For that I apologize. However, I've tried to do a ton of searching online and can't seem to find any answers for the specific issue I'm dealing with. So here's hoping you guys can.
We bought this Tahoe last year. The owner did not have the key fobs. Everytime the door is opened with the key the lights flash and the security light blinks. From what I've read this is normal behavior? The vehicle starts and runs fine so it isn't the anti theft system that kills the fuel.

​​​​​We bought an oem key fob. I have tried to enter the programming mode using the door lock / turning the key 3 times method. However, I cannot get it to enter that mode. I suspect it's the security system but I'm not certain. Would that prevent me from getting into the program mode? If so how do I get it turned off?

The previous owner disabled the horn because of this problem. I want to fix that but figured I'd stay with this so it doesn't honk the horn every time I open the door.

Any suggestions would be much appreciated!

Bob