If your MacBook Air can upgrade further than El Capitan the App Store will recognise this and refuse you El Capitan.
Check these links to see if you can upgrade to Mojave, and if you can't then you might be able to upgrade to High Sierra,
and if you can't upgrade to High Sierra then El Capitan should be available.
How to upgrade to macOS Mojave - Apple Support
How to upgrade to macOS High Sierra - Apple Support
How to upgrade to OS X El Capitan - Apple Support
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
Check that your printers/ scanners etc have updated drivers and software available for the new OS.