DIAGNOSIS F21 is a drain error. The pump ran but the control didn't detect a drop in water level within the expected time. The drain hose or pump is restricted.
FIX First, check the drain hose. Ensure it's not kinked and is less than 8 feet long. The standpipe it drains into must be between 36 and 96 inches high. Then, clean the pump filter. Open the small access door on the front lower-right. Place a towel, unscrew the filter cap slowly, and remove any debris.
IF THAT DIDN'T WORK The drain pump itself may be failing. Listen for a humming sound when it tries to drain. No sound or a weak sound means a bad pump motor (part #WPW10706370). You'll need to access it from the front panel.
HEADS UP Never force the pump filter cap. If it's stuck, turn it gently counter-clockwise while pulling firmly. Forcing it can break the housing, which is part of the outer tub—a very expensive repair.