I have a MacBookAir (2015) running El Capitan. When I search for updates the App Store screen says “no updates available.” On another MBAir (2012) running Sierra, the AppStore screen offers High Sierra. Why would the newer model be unable to update?
If you want to install High Sierra on the MBair, why not just
- download the installer on whatever Mac you can download it
- create a bootable USB installer
- boot from the USB installer and install that way ??