You cannot clean the drain with the dishwasher upright. You need access.
Open the dishwasher. Remove the bottom rack. You will see the cylindrical or mesh filter at the bottom floor. how to clean out a dishwasher drain exclusive
You followed how to clean out a dishwasher drain exclusive guide, but the water still stands. Three possibilities remain: You cannot clean the drain with the dishwasher upright
While you have the machine pulled out, check the drain hose routing. If the hose droops down and then up to the disposal, dirty sink water flows back into your dishwasher. Remove the bottom rack
Exclusive Installation Standard: The drain hose must go UP to the underside of the countertop (creating a "high loop") before coming DOWN to the disposal. Secure this loop with a zip tie.
No high loop = Dirty water re-enters your dishwasher every time you run the sink.