The "F11" is indicating a loss of communication between the Motor Control board and the Central Control board. This error can be caused by a loose wire connection on the machine control board or at the motor control board. It could also mean a failed motor or central control board too. The "F20" fault code means the water level pressure switch has not tripped after 6 minutes during fill. This could me a stuck contact in the water level pressure switch.
The "F21" fault code displays if the drain time exceeds 8 minutes. This could indicate a failed pump, restricted pump, or failed machine control board.
I added some technical information and procedures in the image below. 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
I recommend calling a service technician to diagnose and repair your washer. Call 1-800-4MY-HOME or 1-800-469-4663 to set up a service appointment.