I do not have it checked. I believe is when the scenery references an unexisting asset from a dependency. But even without this asset, the airport still looks right.
I think that what Laminar considers "incomplete scenery installation" should only be applied to when a whole dependent library is missing, not when an asset from that library is missing. After all, assets are added and removed very often from custom sceneries.
If the airport looks bad, then the end user needs to contact the scenery developer, but no flight, specially an online flight, should be paused because an asset is missing..