F21 is a motor problem. If you have already replaced the brushes then you need to reset the error code. To do you need to.
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.
Have you tried it a few times?
As sometimes it can take a few trys before it clears.
Ok if its not clearing it that way then i will opt out and open the question to the other experts and see if any of them can help you on this.