If this is a Kenmore Elite front loader series washer, "F21" is a Long Drain error code. This may be an indication that you have an obstruction somewhere in the drainage system. Most commonly, this means there is something lodged in the drain pump or inlet line running from the wash tub to the pump. There is a white screw off cap on the drain pump that you can remove to inspect the pump impeller and clean. The following link explains:http://www.fixya.com/support/r3608588-f21_error_code
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 and small articles of clothing such as socks or under garments can commonly clog in this area as well causing the drain cycle to fail.
NOTE: A "SUD" error code can often occur in conjunction with the "F21" error code. This is due to air being detected in the drainage system, which can occur if the pump is not running, or if you have an obstruction BEFORE the pump. You need to remove the inlet hose from the pump to ensure it is also not blocked. Correct your drainage system fault, and the SUD error (if displayed) should go away.
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. If the pump requires replacement, the following link explains how:http://www.fixya.com/support/r3574350-replacing_the_drain_pump_in_a_front_load
If you have any questions about this repair, or need additional assistance, please let me know. I hope this helps you.