You just ignore the message.you would get the same thing if you flew a Class 3 flight with a class 2 plane, which is allowed by Euroharmony.
You can fly any class 1 to 3 flights with any class 1 to 3 aircraft and any class 4 to 6 flights with any class 4 to 6 aircraft.
However, you cant fly a class 7 flight with any other class aircraft. You also have to follow the correct division(eg. Eurocargo flight with Eurocargo aircraft, not another division aircraft) and make sure the flight is not too long for the plane.
So unless you are flying a class 4 and above flight with a prop(class 1 - 3), just ignore the message
Hope this helps;D