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F21 is listed as a critical fault.
Either the motor or the main control board (could be both) has a serious problem.
However, it can also occur if there are spikes in the mains and it upsets the control board.
Try clearing the error code table and reset the control board.
Turn the knob to the off position.
Wait 2 seconds.
Turn the knob to the 6 o'clock position.
Wait till the start light flashes.
Press and hold the spin button.
While holding it, turn the knob to the 7 o'clock position.
Release the spin button.
This should clear the error code table.
Turn the knob to off.
Turn on and try the machine again.
Try this a few times.
It should clear the fault code but if the control board or motor is still faulty, it will detect and show the code again.
Hope this helps
Let me know how it goes or if you need more help