The F21 error code indicates a long drain. Here are some tips that should help you resolve this problem:
Check your washer drain to make sure that it is draining properly.
Check the drain hose for kinks and clogs. Make sure that it is installed properly per the instructions in your owner's manual. An improperly installed drain hose can prevent the washer from draining properly.
Check for excessive suds in the washer tub. Excessive suds can prevent the washer from draining properly and will result in the F21 code. Make sure that you are using the proper amount of HE (High Efficiency) detergent to prevent excessive suds.
Place a shallow pan on the floor beside the washer and then pull the drain hose out and place the end of it in the shallow pan to see if the washer will gravity drain. If it does, then excessive suds or a drain pump problem could be causing this failure. If the water does not drain or drains extremely slowly, you could have a clog in the drain system of the washer. Follow these steps to check for a clog in the large item filter of the pump:
Unplug the washer.
Remove the bottom front service panel as shown in the first image below.
Since the washer door is likely still locked, you can manually unlock it by carefully reaching up along the front panel and pulling down on the manual release tab of the door lock assembly until it clicks. You can then open the washer door.
Remove any clothes and then use a wet/dry shop vac to drain any water out of the washer.
Arrange shallow pans and/or towel in front of the large item filter shown in the second image below to catch residual water when you remove this filter.
Remove the large item filter as shown below. It is likely that you will find a clog in this area that is preventing your washer from draining.
If you do not find any clog or restriction in the large item filter and none of the other issues discussed above apply to your situation, then you could have a failed drain pump. If the pump is damaged and will not pump out water then it will need to be replaced.
These tips should help you resolve the F21 error code issue in your washer. If you need more help, resubmit your question with additional details.