I believe this is due to the submit incident component not having the classification category available on the configuration tab under inputs. you will need to use the Submit Incident Advanced component to have the classification field available.
What you can try is adding a single value mapping component before the submit incident component and choose servicedeskincident as the target type. click the mapping definition elipsis and map the output variable that you chose for the classification on the form to the correct classification categories on the right hand side.
This is just a theory but it should work.