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.
How to Fix It
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.