Well, looking at the screenshot, the problem is clear: you are trying to activate "GSX Level 2 Expansion", likely using a GSX Pro key, so it won't work. To activate GSX Pro, you must select GSX Pro.
A common source of confusion is having inadvertently hidden all MSFS product, because you clicked on the MSFS logo when it said "Click here to HIDE products", you clicked there, and all MSFS products have been hidden ( so the icon now says "Products are hidden" ), so you only see FSX/P3D products, and you can easily confuse GSX/GSX L2 for FSX with the MSFS version (GSX Pro), because you don't see anything for MSFS, because they are hidden.
Just click again the MSFS logo, and the MSFS products will appear again, with GSX Pro being on top, impossible to miss...
