You own a discontinued product (Oct 2017) that is not compatible with macOS Big Sur. That means you either subscribe to the current, suitable Microsoft 365 plan, or single-purchase Office 2021 for Mac. If you are sharing Microsoft documents with co-workers, or a publisher, then I recommend you do this. Microsoft does not observe lifetime product purchases once you move from those systems the product was originally supported.
Alternatives to Microsoft would be the free LibreOffice Suite (Office Clone), though it does not work like MS Office, and has a learning curve, or SoftMaker's Office 2021 for Mac which has a subscription or single-purchase plan with a more current MS Office interface. It also has a free trial.
And finally, there are the free Apple's Pages, Numbers, and Keynote which are not Microsoft clones, and attempt translation from Microsoft documents to Apple's own internal document formats, or translation again when exporting. Apple does not guarantee the accuracy of these translations and the document may appear quite different than how it appeared in MS Office.