New units may only be built on a city.

Aircraft must be placed on an airbase.

You must place an airbase on grass or desert.

You can’t place an airbase in a city

That hex already contains an airbase.

There is too much strength in that hex.

There are too many units in that hex.

That hex is out of supply.

You don’t have enough money to build that.

You don’t own that hex.