Can't drop an airport

Solution 1:

I just made an airport, and I struggled a bit myself. But you need the road where the house of the airport is, on the side, (either side) cause it will switch sides to where you put the road.

Not a road all the way along side the runway, but the terminal section ;)

Solution 2:

You cannot place most (if not all) buildings and other ploppable items in an open area, it must be connected to a road.

The snap points it's referring to are the little blue dots that appear next to roads or other structures where you may place it. The road typically has to be at least as long as the edge of the building you are placing and the lot big and flat enough to be placed. Otherwise you will not be able to place the building.