I cannot use Microsoft365 on my MacBook Air, mid 2011, runninc macos Yosemite. What can I do?

Though LibreOffice, a clone of MS Office, still supports Yosemite for the current version, expect that to change at some point. The currently supported, 64-bit Microsoft 365 suite presently requires macOS Mojave 10.14.* for installation, and that is one upgrade beyond what your 2011 Air can upgrade too - macOS 10.13.6 (High Sierra).

Might be time to start thinking about a new(er) Mac with a supported operating from Apple and third-party vendors.

What you have here is an example of some problems that can occur when the operating system in an older computer is never updated. Vendors will continue to update their software and eventually stop supporting their older versions as Microsoft has done. You need to check to see if your computer will accept an upgraded operating system that will run 64 bit apps as well as your old 32 bit apps. If so, then after an update you should be able to install Microsoft 365. If not, then look at installing an app like LibreOffice. It can open Microsoft files and save as when necessary. It is a fine program and has many of Microsoft Offices features.