shffru, The F21 error code indicates a "long drain time" If the washer doesn't drain and the pressure switch does't reset to empty, the door wont unlock.
check and make sure the water level in the tub is below the door opening, unplug the power cord, remove the lower access panel(you should find the tech sheet attached to the panel) reach up between the front panel and the front of the tub, you should feel the manual release plunger, pull down on the plunger, this will unlock the door. You can then check the drain pump and hoses for clogs, debris, etc. You can also use the tech sheet to review the error codes and run the diagnostics service mode.