As explained so many times, on the forum, the manual, and everywhere else, you should never post questions about orders on the public forum because, of course, the next step here would be asking details about your order which cannot obviously discussed in public. So you should always use the support EMAIL (The "Contact Us" page of this site) for any questions related to the activation.
The only generic answers I can give here are:
- If you have consumed all your activations, you will NOT see a message saying the key is invalid, you'll see an unmistakable different message saying you don't have any activations left, so you must use the Self-Help site (The FAQ page of this site), to DEACTIVATE older activations to make space for new ones.
- If you see an error message saying your key is invalid, it's because IT IS invalid, which usually is caused by one of these reasons:
1) You mis-typed it. Type it correctly.
2) You typed it correctly, but selected a different product to activate. Most common mistake is trying to use a key for an FSX/P3D version of an airport to activate the MSFS version. They don't share licenses, with the only exception of LFSB.