Almost everything in the airport (except the runway and taxi links) are covered with "dirt" surface, making it impossible to properly taxi.
After inspecting AFX_FSIA.bgl, I see that you've placed a "fake" apron, covering the entire airport, exactly to prevent this. Unfortunately, it doesn't work for me for some reason.
Editing the AFCAD in ADE doesn't help either, as it breaks a lot of other things (I'm aware that AFX is your preferred tool, but I have no idea how to use it).
![Image](https://i.imgur.com/hZElXNW.jpg)