What does the F21 error code mean and how do I resolve it?

The F21 error code indicates a drain problem. Press PAUSE/CANCEL twice to cancel the cycle and unplug the washer, then check whether the drain hose is kinked, frozen, clogged, or positioned more than 96 inches (2.4 m) above the floor. Plug the washer back in, re-select the cycle, and press START; if the problem persists, call for service. (Page 19)