DIAGNOSIS That's the door alarm. It's triggered because the magnetic door switch thinks the door is open, or because the unit's internal temperature is way too high, usually right after you plug it in.
FIX First, make sure the door is completely, firmly shut—the seal needs to click into place. If it's still beeping, unplug it for a full 5 minutes. Plug it back in and immediately close the door. It needs to cool down internally before the alarm will shut off, so give it 10-15 minutes. The beeping should stop once the interior temp drops.
IF THAT DIDN'T WORK Check the little rubber door gasket. If it's pinched or folded over, it can block the door from fully engaging the switch inside the frame. Straighten it out all the way around.
HEADS UP The alarm is LOUD by design. If you need to move it or leave the door open for cleaning, there's usually a button combo to disable it temporarily—hold the 'Light' and 'Up' arrow buttons together for 3 seconds. It'll re-enable next time you power cycle.