Some microwave models have a built-in 5-minute internal timer as a safety feature preventing the microwave from cooking without food inside, since doing so can cause the unit to overheat.
From a google search on your issue.
If food is placed into the cavity before selecting a cooking sequence, opening and closing the door activates the switch, initiating the 5-minute internal timer. The door latch switch must be opened and closed before hitting “START,” which signals the unit that food has been placed inside. The "FOOD" or "Insert Food" message on the display means the microwave thinks food has not been added yet. To correct the issue, open and close the door again and press “START.”