"F21" is a Long Drain error code. This is an indication that you have an obstruction somewhere in the drainage system. In most cases, this means you probably have a clog in the drain pump. The following link explains:http://www.fixya.com/support/r1484065-f21_error_code_whirlpool_duet_kenmore
You may have something getting caught in the pump impeller and slowing it down. Included is a follow-on link that explains how to check the drain pump to clean. You may also have to remove the inlet hose that runs from the bottom of the wash tub to the inlet side of the drain pump. Debris can commonly clog in this area as well causing the drain cycle to fail.
NOTE: If these preliminary measures do not correct your problem, you may have to replace the drain pump. It does not take much to damage the pump impeller or motor. Since the error code is generated if the drain cycle exceeds 8 minutes, if the pump is damaged and can't pump at the required volume, the error code will continue to appear until the pump is replaced.
NOTE: The "SUDS" error can commonly occur with the F21 error code as this is can be generated when air is sensed in the drainage system.
If the door won't open it could be jammed due to the program being interrupted.
If you have questions about this repair, or require additional assistance, please let me know. I hope this helps you.
NOTE: If replacing the drain pump is required, the following link explains how:http://www.fixya.com/support/r401845-replacing_drain_pump_whirlpool_duet
For troubleshooting guidance, the washer comes with a technical data sheet. It should be located behind the lower toe panel directly below the door. If not located behind the panel, it will be affixed to one of the interior cabinet walls. The tech sheet includes diagnostics procedures, troubleshooting tips and the error code list.