After looking up on the internet what the F21 error was, a possible obstruction in one of the hose lines. We took every hose off the washer and cleaned them all out. We did find that when we removed the drain pump cover, it was full of various items from years of washing clothes. After a very through cleaning, we put everything back together and tested the washer. Again, an F21 error.
After more research, the next item would be the drain pump itself. I bought a new drain pump, installed it, and we are back to washing clothes!!