DIAGNOSIS The control board has lost communication with the keypad/touchpad unit. It's a common internal wiring or connection fault.
FIX Unplug the microwave for 5 minutes. This is the most likely fix and resets the control board. Plug it back in. If F-11 clears, you're good. If it returns immediately, the internal ribbon cable connecting the touchpad to the main board is likely loose or damaged.
IF THAT DIDN'T WORK The fix requires internal access. You must remove the outer case to check the flat ribbon cable connection between the touchpad membrane and the main control board. Reseat it firmly. If the cable is damaged, you need a replacement touchpad assembly.
HEADS UP Disassembling the microwave involves high-voltage components even when unplugged. The capacitor holds a dangerous charge. If you're not experienced, this is a job for a technician.