I am not 100% sure about this but if it does not work I am afraid you will have to call the service guy €€€€€€€€!!!!!
F21 relates to a motor or control board fault.
However, it is also caused by spikes in the mains and can be spurious.
To reset it.
Turn the appliance knob to the off or reset position, 12 o'clock.
Turn the knob to the 6 o'clock position.
Press and hold the spin button.
Whilst holding it, turn the knob to the 7 o'clock position.
Continue holding the spin button for about 5 seconds.
Release the button.
Turn the knob to off.
When your turn it back on it should have cleared the error code.