In most cases this error is caused by a pinched or stopped up water level pressure switch hose. Remove the top off the washer and examine the pressure hose for kinks, clogs or tears. The hose is attached to the pressure switch and the washer tub. (See video below to get an idea of what you are looking for)
If nothing is found wrong with the pressure switch hose then you will need to do a pressure switch test.
See Whirlpoool Duet Washer Water Pressure Switch Test