The F21 error code indicates a long drain time on this type of washer. The door will not unlock if the washer control detects that water is still in the tub. To manually unlock the washer door, take these steps:
Unplug the washer to completely disconnect electrical power.
Remove the bottom front service panel as shown in the first image below.
Carefully reach up through the bottom opening and pull the release tab on the bottom of the door lock assembly until it clicks. NOTE: Internal metal edges in the washer can be sharp. Protective gloves are recommended for this procedure.
By pulling the manual release tab on the door lock assembly, you should be able to get the washer door open.
After opening the door and removing the clothes, I recommend emptying any remaining water in the tub with a wet/dry vacuum (or bail it out). Since you already have the bottom front service panel off and the washer unplugged, you can arrange shallow pans and towels to catch any remaining water and pull the large item filter out of the drain pump assembly as shown in the second image below. You may find a clog in this area that is preventing your washer from draining properly. Clear any clog or debris from this area and reassemble the washer. Check your drain hose for a kink or clog. Check the washer house drain for a clog or restriction. Test the washer to see if it will drain properly.
Other issues that can cause this problem include:
Not using HE (High Efficiency) detergent
Using too much regular or HE detergent
These issues can cause oversudsing in the washer. The drain pump will not able to pump out suds. Make sure that you are only using the appropriate amount of HE detergent in the washer. If you have a water softener, you may need as little as a tablespoon of HE detergent.
If you need more help, resubmit your question with additional details and the full model number of your HE2 washer.