DOES NOT SPIN AND DRAIN. INDICATES F21 ERROR MESSAGE

This is from an old posting. Common problem:
I had the same F-21 error and thanks to others who have posted, I was able to solve the problem. F-21 is the code Whirlpool uses to signify that the machine has tried to empty the drum unsuccessfully for four minutes. This is telling you either the hose that drains to the wall is blocked or the filter for the pump is blocked. If your hose is kinked, you have solved the problem. If it is not, then you have to clean the filter. To do this, do just as the others have recommended...
1) Unplug the machine.
2) Remove the front panel(my duet had 3 1/4" sheet metal screws on the bottom of the panel.)
3) Unscrew the white cap you see next to the small little pump.(note, any water in the machine will flood out. Have about 20 towels or better yet...a wet/dry shop vac. Someone else suggested this and it worked great. Once you turn that white cap a revolution or two, water will come out slowly. You can use the shop vac to **** it up before it hits the floor)
4) Pull out the cap completely and look at your new treasure. Mine consisted of the following: Two collar stays, one small plastic jewel from a shirt, two pieces of orange plastic of unknown origin and the big prize...one slimey sock.
5) Carefully replace the white cap being sure not to cross thread the connection.
6) Replace the front cover(this was the hardest part, those small screws are a pain to work with)
7) Plug it back in and fire it up!!!

This error code indicates a major issue that concerns the Microcomputer and Mcu. you will need to replace these two boards to restore proper function.