I can't upgrade to macOS Monterey due to lack of storage

I think your MacBook Air can upgrade to macOS Monterey

Here are the macs that can run Monterey.

macOS Monterey is compatible with the following computers. 

If your Mac isn't compatible, you may be able to install an old version of macOS instead.

MacBook introduced in 2016 or later

MacBook Air introduced in 2015 or later

MacBook Pro introduced in 2015 or later

Mac mini introduced in 2014 or later

iMac introduced in late 2015 or later

iMac Pro introduced in 2017 or later

Mac Pro introduced in 2013 or later

View the complete list of compatible computers.

If upgrading from macOS Sierra or later, your Mac needs 26 GB of available storage to upgrade.

If upgrading from an earlier release, your Mac needs up to 44 GB of available storage.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If you want to upgrade to Monterey open Safari and click on the link below,

How to upgrade to macOS Monterey – Apple Support (UK)

If you are considering upgrading to Monterey be warned it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this free app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.

Tips: Upgrade to Monterey macOS 12

Which MacBook Air do you have, in all likelihood you cannot upgrade the RAM memory

or the SSD in the unit.