Assuming you used the same email to contact FSDT support and to register to this forum, I can confirm no emails from your email has ever been received about this issue. Simmarket only sells GSX, that is the FSX/P3D version so, maybe you wanted to buy GSX Pro instead, the version for MSFS 2020/2024 ?
If yes, it's normal you'll see an invalid key message, because a key for GSX won't activate GSX Pro. GSX Pro is only sold on the FSDT website, so if you really wanted to buy GSX Pro instead, you should ask Simmarket for a refund or store credit, indicating you bought the wrong version by mistake.