The App Store Updates page only shows updates for the Mac OS you are running at the time. If you want to install a newer version of OS that is called an upgrade and they are never listed in the Updates section.
Your 2014 MacBook Air 2014 can upgrade to Mojave, follow this link.
How to upgrade to macOS Mojave - Apple Support
Before you decide to upgrade make a full back up of your Mac. Contact the developers of any third party software you use to see if there are updates available so they will run in Mojave. You will also need to check with the manufacturers of any printers/ scanners etc to see if there are updated drivers and software also.