Well if you have a 2012 MacBook Air you can upgrade it to macOS Catalina. Which will run the latest Office. But not the 2011 version since its 32 bits and Catalina doesn't support 32 bits software anymore.
If it has to be Office 2011. The last version of macOS to support 32 bits software is 10.14 Mojave. But as Noclue751 already mentioned. Office 2011 is no longer sold or supported by Microsoft.
So I fear you have to look for alternatives. Such as Microsoft 365 (which was called Office 365 till very recently), Office 2019 as a single one time purchase or free options like LibreOffice or Apple's own suite.