1st Problem

[Sweet_H3]

I’ve had my truck about 3 months and this weekend I noticed a problem.
My rear tailgate doesn’t lock. I use the keyless remote to lock the truck I can hear the doors latch close but when I try the tailgate it opens. I locked the truck using the controls on the drivers side door again I hear the doors latch but the tailgate still opens and the horn makes a small honking sound for about 30 seconds and then it starts honking very loud like an alarm. I locked the truck again using the remote I checked all the doors they were all locked, I checked the tailgate, and it opens. What’s up with that??