Submit button error as states not available on responses

As you noted, the responses in a Matching Drag and Drop question don't have states. That means there isn't a way for us to check whether the user has answered the question.

Fortunately, question slides have built-in functionality to do that. When a question is submitted, Storyline checks whether the user answered the question. If not, it automatically shows the Invalid Answer message:

That ensures that users answer the question (which would be the point of disabling the custom Submit button until the response is done).

Your slide could also take advantage of other built-in functionality. For example, you don't need any triggers to show the Correct or Incorrect layer. That happens automatically. 

Also, when you put the explanation and symbol on the layers, you don't need triggers to change them from Hidden to Normal. 

And you don't need a trigger to hide the custom Submit button. Because the Correct or Incorrect layer will show after the question is submitted, deselecting the eye icon for the Submit button hides it at that point.

A simplified version of the slide is in the attached file.

By the way, Form View has the responses for the last two items mixed up: