The f21 is one of the most common failure modes of this washer. Nearly always the problem is a clog in the drain path. If some debris gets through the pump strainer basket, it can wind around the pump impeller. Usually by this time there is plenty of junk in the rubber sump hose and the pump strainer. So when you are emptying the basket, look into the drain hole or remove the spring clamp and separate the sump hose to ensure there are no pens or pencils in it.
As quiet as the units are, one of the loudest sounds it makes is when draining. When the pump impeller is jammed, it doesn't make any noise. You have to touch the motor to feel it humming unless you have very good hearing and a quiet spot. While the strainer basket is out, the impeller can be seen through the right hand hole of the cavity. Make sure it isn't jammed or separated from the motor.
Also, the drain hose and pressure sensor can get clogged up with muck. While the cover is off the strainer, pour a little water into the drain hose and make sure it falls out of the strainer hole. Testing the pressure sensor tube can be done the same way. No flow means there is a clog.