According to this article:
In the case of "F21," check the drain hose for kinks, freezing and clogging. Remedy and try the cycle again.
And on this site:
The F21 error on this line of washers is the machines way of saying the water took to long to drain from the tub. This could be a problem with one of the washer parts including the drain pump itself, how the drain hose enters the stand pipe, or maybe even something is stuck somewhere in the drain system and is preventing the water from flowing properly. Not quite an emergency, but it is an indication of a problem and a problem that can lead to something bigger if not looked into.
So it looks like your drain hose may be clogged or kinked in some way. Perhaps you should remove it and inspect.
Hope this helps!