DIAGNOSIS You have a door switch failure. F0 is a door interlock fault code. The main control board isn't detecting the correct open/close sequence from the switches.
FIX Unplug the microwave. Remove the vent grille. Locate the three door interlock switches on the top front of the cavity. Test each switch for continuity with a multimeter while pressing the actuator. Replace any faulty switch. Reassemble and plug in.
IF THAT DIDN'T WORK The wiring harness connector to the main control board can corrode or come loose. Check the harness for damage. Reseat the connector at the board.
HEADS UP Do not bypass the door switches. This creates a radiation hazard. The unit will not operate with a confirmed F0 fault for safety.