Diagnosis
That's a door switch error code. The microwave thinks the door is open or the safety interlocks aren't engaged, so it refuses to run for safety reasons.
How to Fix It
Unplug the microwave. Check the door for any obvious food debris or gunk blocking it from closing fully. Look at the little plastic latch on the door edge and the corresponding slot inside the cabinet. Clean both areas thoroughly. Plug it back in and try closing the door firmly. The error should clear after a few seconds if the switches are making contact.
If That Didn't Work
One of the three door interlock switches has likely failed. They're mechanical switches that get a lot of wear. You'd need to open the cabinet (UNPLUGGED!) to test and replace the faulty switch. It's a common repair but requires careful work due to the high-voltage capacitor.
Heads Up
Never try to run the microwave or bypass these switches. They're critical safety devices to prevent radiation leakage when the door is open.