"F21" is a Long Drain error code. This indicates that you have an obstruction in the drainage system. More than likely 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
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.
If you have any questions about this repair, or need additional assistance, please let me know. I hope this helps you.
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.