DIAGNOSIS That's a control board communication failure. The main board isn't talking to the touchpad board properly, usually because of a loose connection or a failed board.
FIX First, unplug the microwave from the wall outlet for two full minutes. While it's unplugged, press and hold the number pad buttons for about 30 seconds to drain any residual power. Plug it back in. If the 'FF' is gone, you're good. If it comes back, you'll need to check the wiring harness between the two boards, which requires removing the unit and its housing. For safety, I'd recommend getting a technician for that internal check.
IF THAT DIDN'T WORK If the code persists after the power drain, one of the control boards has likely failed. The main control board is the more common culprit. A service call is your next step.
HEADS UP Don't ignore this code. The microwave is in a lockout state for safety and won't function until the communication error is cleared.