I can't download app or updates on MacBook Air

Sorry, but if an app requires a version later than 10.13.6 you won't be able to use that specific app. There may be alternatives such as an earlier version of the app or a similar app.

If you have specific apps in mind, you may want to reply with the name(s) of the app(s) and someone here may be able to provide more information.

One option to consider is Libre Office. It is free and is very good and requires macOS 10.12 or later. See: https://www.libreoffice.org/get-help/system-requirements/#Apple and https://www.libreoffice.org/

Also see the following from Microsoft's support forum: https://docs.microsoft.com/en-us/answers/questions/242074/unable-to-install-office-in-macos-high-sierra.html

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.