Project says you need to add Post Processing V2, and you do this, then it says to clear the light-maps. After you clear the light-maps you go back to window, and re-try to use Ambient Skies and yet again it asks you to do the same thing. What is happening is you're project does not have the line of coding that it needs. So to fix the issue you need to go to you're project settings. Then go to PLAYER, Scripting Define Symbols and make sure that:
is in that section. If not please add this to the section by typing this in. This is a Unity Bug, that does not catch the right scripts that needs to be added. After you are finished typing this in please hit enter or it will not save!
Please See Attached Picture Below: