I have 28 GB free space on my MacBook Air but can't install macOS Monterey

From: How to upgrade to macOS Monterey - Apple Support

"If upgrading from macOS Sierra or later, your Mac

needs 26GB of available storage to upgrade. If upgrading from an earlier

release, your Mac needs up to 44GB of available storage."

You need more free space.

You have to free up space. You do that by deleting user data (documents, movies and such) after copying it to an external destination such as an external hard drive or a cloud storage service. iCloud Drive is not a long term autonomous storage solution.

Once copied and verified, delete those copied files and folder off your internal drive. Do not delete any system files or folders.