Starts but does shuts off after few seconds?

I'm assuming it has to do with the pass lock
for the security. I tried to do the 30 minutes m relearn, but it didn't work. When trying to reset my security light comes on the first turning to on position then goes out a second or so later, then comes back on and stays on. No matter how long the light doesn't go out. In the reset it says it will blink stay on then go out after ten minutes and then turn the key to off position 10sec back to on until light goes out again reapt. Etc. But mine just stays on. I even left it on for over an hour because I heard will reboot or Sumin after left in position for hour. Super frustrating, I've been without a vehicle for over 6 months. My buddy's mom lends me this extra van she has. We drove it up to Washington from OR. No problems, gas gauge doesn't work so you just have to estimate mileage. I fig it was getting low since the last fill up but was super tired, so I parked it at home instead of getting gas that night. The next day came out and the battery was drained because the stereo got left on, I think. And it's done this no running thing since then. I went and put like 4 more gallons in tank just to make sure it wasn't out and still just does the start and shut off. Just some more info on back story. If it helps at all, lol. I originally thought the security got tripped cause of the drained battery maybe, but this is my first GMC and pass lock nightmare, lol I don't think I have a transponder key, so it would be in the ignition correct? This is the key.