I recently got a MacBook Air and every time I try to download an app from the Mac App Store, a message box appears saying: We could not complete your purchase. (null) can't be installed on "Macintosh HD" because OX X version 10.12 or later is required. And I look at the Updates tab and there are no updates available. I am certain my MacBook is up to date so why can't I download anything?
Edit: My Macbook is OS X Version 10.8.5