The problem is caused because you have a *DEFAULT* flight plan (made by selecting Departure and Destination on the Main menu) loaded, which doesn't match the one on Simbrief because, while if there's no default flight plan active, for GSX to consider a Simbrief flightplan as valid it's enough the departure airport matches the one you are located, if there IS a default flightplan active, BOTH Departure and Destination must match the ones on Simbrief, and in this case they don't, because they are the same.
The most common reason for having a flight plan with the same airport as departure and destination, is having selected a Destination in the Main menu map first, then a Departure on the same airport without realizing it, and this creates a flight plan.
