Whirlpool Model # WFW9200SQA12 error code F-21 means machine has water draining issue or slow drain. The control has noticed that the time to drain the water out of the washer is longer than normal.
The first thing to check would be your drain pump trap. Unplug the washer first before you start. Remove the three screws located a very bottom of washer and remove bottom panel. Now you have a clear view of the washers drain pump and trap. Remove as much water from the tub as possible before moving on to the next step.
Turn the trap cover counter clock wise half a turn. Make sure you have a shop vac ready there will be water in the drain pump. Slowly remove the cover and clear drain trap of all debris. Reinstall the trap cover and put machine into drain spin. You should hear the pump working at this time and machine should go into full spin.
If you do not hear your pump turn on at the start of the cycle you my have a bad drain pump. Check to see if your drain pump is getting voltage, your looking for (120v). If you are getting voltage to the pump you will need to replaced the drain pump motor. Part # W10130913