My Macbook Aiirs is short 20 GB to install Monterey IOS

First, please review Free up storage space on your Mac - Apple Support and Optimize storage space on your Mac - Apple Support.

If you are still unable to create sufficient space for the upgrade after exhausting all those steps, the workaround is to create a Time Machine backup, completely erase the Mac, install the latest version of macOS (read • below), and finally transfer the contents from that Time Machine backup when asked. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.

• To install the latest macOS that is compatible with your Mac, refer to Other macOS installation options in How to reinstall macOS - Apple Support.