I need to upgrade my system from 10.13.6 to 10.14, is this available?

That depends on the model year.

The model year of a MacBook Air is available at Apple menu > About This Mac.

A mid 2009 MacBook Air can be upgraded to El Capitan 10.11.6 but no later version. See: https://support.apple.com/en-us/HT211683

A late 2010 through mid 2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version. See: https://support.apple.com/en-us/HT211683

A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later. See: https://support.apple.com/en-us/HT211683

A mid 2013 and early 2014 can be upgraded to the latest version of Big Sur macOS 11 but no later. See: https://support.apple.com/en-us/HT211683

An early 2015 or later can be upgraded to the current version of Monterey macOS 12. See: https://support.apple.com/en-us/HT212735

feelord wrote:

My computer is a2017, Mac OS ver 10.13.6, can I upgrade to 10.14

Yes.

Tap or click on the following blue-text link for detailed instructions around how to upgrade your Mac ➡️ How to get old versions of macOS - Apple Support

Have a complete backup prior to performing the upgrade to macOS 10.14 Mojave.