You are correct f21 error code is a drain problem. If you select a drain and spin cycle and the drain motor runs but will not drain, the filter may need to be cleaned. I have provided Pump replacement instructions in the image below. If you still get an f 21 error code with an empty tub, it is most likely a pressure switch failure. Remove the top and locate the pressure switch. Remove the pressure hose and blow into the hose to insure the hose is clear. Now check the electrical connections on the pressure switch. With the pressure hose reconnected start a drain and spin cycle. If the f 21 error code still comes up, I recommend replacing the pressure switch. Any parts you need can be ordered from this link.
Sears Parts Direct