Clogged or Improperly Installed Garbage Disposer
If the garbage disposer was installed recently, the drain plug may not have been removed. All garbage disposers are sold with a solid plug in the drain tube. If the dishwasher is not connected to the garbage disposer, the plug must remain in the drain tube. However, if the dishwasher is connected to the garbage disposer, the plug must be removed. If the garbage disposer wasn't replaced recently, it is possible that the disposer is clogged. Remove the dishwasher drain hose from the disposer and clean out both the fitting on the disposer and the end of the drain hose. These are the most common places to find an obstruction if the dishwasher won't drain.