Whether you Mac can upgrade past Big Sur depends on the exact model of your Mac. You can get the exact model by clicking the Apple menu and selecting "About This Mac". You need a MBAir 2015 or later model to run macOS 12.x Monterey.
FYI, macOS Monterey will no longer be supported by Apple in a couple of months once macOS 15.x Sequoia is released. Microsoft seems to be following Apple by only actively supporting the three most recent versions of macOS. If you happen to have Office installed, then it will continue to work even after MS stops supporting it, however, if you use any online services with those apps, then sooner or later they will stop functioning once the online component changes over time. Plus if you receive documents made with a newer version of Office using newer features or document formats, then you won't be able to open or read them with an outdated copy of the software.
Also, MS Office is not free....only the updates to the specific version of the Office apps are free.
If you want free software for accessing MS Office documents, then checkout LibreOffice or use the Apple apps Pages, Numbers, and Keynote (I believe these Apple apps may have been installed from the factory, if not then it is unlikely you will be able to download them from App Store now).