Diagnosis
The door safety switch isn't engaged. The control needs a signal that the door is fully latched.
How to Fix It
Unplug the unit. Open and close the door firmly. Listen for a solid click from the latch mechanism at the top center of the door frame. Check that the metal latch hook on the door itself isn't bent. Plug it back in and try.
If That Didn't Work
The door switch microswitch inside the top housing might be faulty. You'll need to remove the top cover to inspect it. Look for a small black plastic switch with a lever that the door latch depresses.
Heads Up
Never bypass the door switch. The appliance will not operate without it for safety reasons. If the switch is bad, you need to replace it.