I also saw , steam created a -1 ID for the needed Planes , that i have fixed. So it should work now

I just talked with Joscha (Developer of SA) , he told me it is a bug in the Skybus mod.(not my mod) , that have a steam ID -1 (that not works). You just need to unsubscrbe that Skybus Mod. It could be , that your old save not worked anymore because of that mod. BUT: you can fix that Skybus mod , open it with notepad++ and change the Steam ID -1 to the real ID , then it will work again. But that Skybus Mod creates the problem. More i can not help

Unsubscribed and resubscribed, error still exists.
Trying to start a game with *no* airlines activated except this Air Vietnam.

The games "missing mods" popup is trying to link to an Airline mod with id 4294967295
But the only SkyBus airline on steam has id 1640804363 and it doesn't show in the list of available airline (unless it just has a different display name)
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1640804363

Perhaps there used to be another SkyBus airline that got deleted off steam?

(PS, just tried your other Air Vietnam and getting the same error with that one too)

That Airline works at me. If it wants a other airline , then the steam vdf is broken(steam download) . Unsubscribe that mod , and subscribe it again. That it should work + add the needed plane

bugged, requires an airline named skybus but not the one on steam