how cant update my macbook air

Is it possible the term update is being mis-used to mean upGRADE? If upGrade is meant - it will be governed by the Age of the computer as to what version of macOS it Qualifies for. In the link supplied you will have to check what version of macOS it qualifies for. Open the links in next page using Safari as Others may not work.

What happens when you try updating?

How to update the software on your Mac