Can you receive a DUI if you are sitting in your car in your parking lot and the car is not on?

If your keys are in the ignition, yes. Also, if the policeman saw your car moving prior to him issuing the ticket, then yes.

id like to add.. Yes most likely, The only way you can't is if the car is locked, and you dont have the keys... But i have heard people get away with it by pacing down the road fifty steps or something and hiding your keys leaving your car unlocked, the excuse is that you've lost your keys, so you drank some beer and plan waiting to be picked up or sleep the night or something because its unlocked.