My MacBook Air is not supporting Microsoft

If by not supporting Microsoft, you mean that Microsoft Office apps will not run on it, be aware that Microsoft does not provide free upgrades between major versions of their Office products (unless you subscribe to Microsoft 365) and older versions of Office are not compatible with macOS Catalina or Big Sur.

If you have a version of Microsoft Office that is not compatible, you can either subscribe to Microsoft 365 or you can make a one-time purchase. See: https://products.office.com/en-us/buy/compare-microsoft-office-products-for-mac?ms.officeurl=mac-compare-redir

Or use the free and very good Libre Office https://www.libreoffice.org/

or the free Microsoft Office on the web: https://products.office.com/en-us/free-office-online-for-the-web

or Google Docs: https://www.google.com/docs/about/

or Apple’s Pages and Numbers