Big Sur will not load on Mac air 2014, shortage of space

See the following from How to upgrade to macOS Big Sur - Apple Support

If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Big Sur requires up to 44.5GB of available storage.

The following may help you free up space: How to free up storage space on your Mac - Apple Support