MacOS
How can you install a newer version of iOS on late 2011 machine. Office 365 won't work unless it has iOS 10.15 or higher?
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Macs don't run iOS. Macs run macOS. A late 2011Mac is not capable of running a version of macOS that will run any supported version of Microsoft Office.
You will need a new machine or you will have to find an alternative office suite.