The first thing I would check is the air gap on top of the sink (if any).

Remove the cover from the air gap and the top piece and check for any clog.

Remove the drain hose from the air gap, put it in a bucket and run the drain cycle. Check for any foreign objects coming out.

If there is nothing wrong then you have to remove the dishwasher from the cabinet, remove the drain pump and check for any foreign objects inside the pump and/or hoses.

Check if the drain pump flopper (red piece) is in place.