This article describes the reason why some airports might be missing in OnAir or your simulator.

Because the OnAir database is as inclusive as possible, there are some airports that are not in every simulator or add-on. It’s like in real life, sometimes, when flying non-regular lines you have to land at some airports without any data in your GPS, navigation or Flight Management System.

When you select a job, you have to be sure to have the correct add-on (specially for tiny airstrips) and the documentation for landing safely. To overcome these issues altogether, you can send AI to fly jobs to/from airports that you do not have available in your simulator.

Furthermore you can also turn off “Use small airports” in the Global Settings and jobs to/from size 0 to 2 airports won’t generate for you (those are the most common airport sizes that aren’t in every sim and Navigraph data).

If you use XP11 then you can also try to add the airport using the Prefab Airports pack

https://forums.x-plane.org/index.php?/files/file/27582-prefab-scenery-for-25000-airports/