F21 is a motor problem.
What you can try is this.
Turn the knob to off.
Close the door.
Turn the knob to 6 oclock position.
Wait 2 seconds.
The start/pause should begin flashing.
Press and hold the spin button.
Turn the knob to 7 oclock position.
Let go of the button.
This should clear the error code and retry the washer and see if it goes again.
If it does then you need to check the carbon brushes in the back of the motor and check to see if they have worn down. If so then they will need to be replaced.
Then retry the washer and see how it goes from there.