Save/Reload works wonders mostly.
- Does your airport have vehicle depot?
- Did you try to save and reload that save game?
- If the 2 options above didn’t work, press F10 to oen debug menu and click “unoccupy all road nodes”
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I think that Depot is a pre-requisite to order any type of vehicle… 
He could have (re)moved the depot are have one that is inaccessible
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So I had the depot and there were some cars that went there before. I reloaded the save and it worked. Thanks!
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If you run the game for longer periods of time some pathways become broken and reloading the game fixes it. Vehicles have the ability to override the broken pathways to an extent but that doesn’t mean the pathway is not broken for the next vehicle trying to use it.