I have 6 small and 5 medium gates but nothing larger and the game wants to land a large aircraft. Is this a bug? Nothing like this has happened before
Maybe you reached a level where a large plane comes as an achievement.
I took a screenshot at the time. Looking back through the achievements it looks like this may be the case although I think the game should wait until I have a large gate!
R14 and R15 are L stands you can build without a terminal. Maybe you build one of them for this plane.