Diagnosis
F21 means "Long Drain"—the washer tried to drain but didn't see the water level drop within the expected time. It's a drainage problem, not a fatal error.
How to Fix It
First, check for a simple drain hose kink. Pull the washer out and make sure the drain hose going into the standpipe or laundry sink isn't pushed in too far (more than 6 inches can cause a siphon issue) or bent. Straighten it out. Also, clean the little filter at the end of the drain pump hose inside the machine. On this model, it's behind the lower front panel. Unplug the washer, open the panel, and you'll see a small hose with a cap. Have a towel ready, unscrew the cap slowly, and clear any debris.
If That Didn't Work
The drain pump itself might be clogged with a sock, coin, or bra wire, or it might have failed. You'll need to check the pump impeller for obstruction or test it for continuity with a multimeter.
Heads Up
Don't ignore this error and keep trying cycles. If the pump is truly dead, you'll flood the tub and potentially cause a bigger mess.