Microsoft has retired every earlier Office suite except their current Microsoft 365 (subscription), the current Office 2021 for Mac, or the preceding Office 2019 for Mac (both single-purchase). All earlier Office suites have had their license servers disabled so they cannot be registered for any new installations. Existing installations of Office 2011 for Mac will continue to work on macOS Mojave or earlier, where 32-bit application suites are supported.
Your Early 2015 MacBook Air certainly can be upgraded all the way to macOS 12.* (Monterey), though after Mojave (10.14.*), only 64-bit applications, printer, and scanner drivers are supported. The free Go64¹ application will show you which of your current applications are 32-bit, and an opportunity to verify with third-party vendors if they have Catalina or later compatible 64-bit applications available. You will also need to verify with your printer/scanner vendors that they provide 64-bit driver support (released in 2021) for the intended operating system upgrade choice.
You have a link from Jack-19 on how to upgrade your MacBook Air.
¹ Go64 is a self-contained application that does not spew content elsewhere on your Mac, making it easy to remove by just dragging it to the Trash when you are done with it.
