Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
If you can't upgrade the operating system on your Mac, you won't be able to run Microsoft Office. See https://www.brandwares.com/bestpractices/2023/03/most-recent-office-you-can-run-best-practices/ . The requirement is macOS 12 or later and an Office product which licenses version 16.83 or later (Microsoft 365 subscriptions will meet the latter requirement).
As an alternative, you can use LibreOffice which is open source software. Compatibility with Microsoft Office formats is generally good.