Fact you are referring to GSX and said it's an MSFS install, might suggest you are confusing GSX with GSX Pro, perhaps, a most common mistake many users do when activating or reactivating, assuming the "key doesn't work", because they are using a valid key on the wrong product.
Of course, I can't say more without checking exactly about your error message, which will show exactly what you typed and what you are trying to activate. And that's why you should never post these kind of questions on the support forum but, instead, using the support EMAIL because, now my next question would be checking your order, you error and your key, which we can't of discuss in public.
