The model year of a MacBook Air is available at Apple menu > About This Mac.
A late 2010 through mid 2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version. See: How to download and install macOS - Apple Support
A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later. See: How to download and install macOS - Apple Support
A mid 2013 and early 2014 MacBook Air can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download and install macOS - Apple Support
I checked the Mac App Store (Apple menu > App Store) and it indicates that Slack requires macOS 11 or later so if you have a 2013 or later model MacBook Air you should be able to use the above instructions to upgrade to that.
Be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.stclairsoft.com/Go64/
or
https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html