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The "F21" error code indicates it takes too long for water to pump out of the tub. This can be caused by using too much High Efficiency detergent or using too much of the wrong kind of detergent. This error code can also be caused by debris clogging the drain pump or a faulty water level pressure switch.
If the fault code reappears after cleaning the pump filter and by only using 1 tablespoon of detergent, then I would recommend calling a service technician to diagnose and repair your washer.
NOTE: Disconnect the power cord from the power source before removing the lower access panel and cleaning the pump filter
. Be sure to have all the water pumped out before removing the pump filter cap. You also will need an old large towel to soak up residual water once you remove the filter cap.
I am including a couple of images below that demonstrate gaining access to and clearing the drain system.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services
Here is a link that you may use to view the parts list diagram and for parts purchases;
Sears Parts Direct
I hope this is helpful. If I may be of further assistance as more details become available, please reply to this post.