The first thing I would check is the air gap on top of the sink (if any).
Remove the cover from the air gap, remove the top piece and check for clog.
Remove the drain hose from the air gap or wherever it is connected, put the end of the drain hose in a bucket, run the drain cycle and look for any foreign objects coming out.
If this is not the issue, then remove the bottom panel, remove water from the dishwasher, disconnect the drain hose from the check valve (#15 on the
pump & motor break down diagram) and remove the check valve (turn CCW).
Inspect the check valve for a clog.
Here are the break down diagrams for the Whirlpool dishwasher
Model DUL200PKQ1
Gene.
P.S. There is no drain pump in your dishwasher.