Xcode can’t be installed on “Macintosh HD” because OS X version 10.12 or later is required.
That is the error message i get from app store when i try to download it. mine is also 2015 macbook air
I'm running MacOS Sierra 10.12.4 (I haven't updated my OS in a while). If you have the 2015 model you should also have MacOS Sierra installed. As JLBorges said, try to download the software updates for your Macbook Air to the latest version. I usually just update it from the Appstore.