Why cannot i activate my purchase of Microsoft word

Your license key for Office 2011 for Mac, or the old subscription license will not work in current versions of Microsoft 365, or Office 2019 for Mac. If you purchased Office 2019 for Mac, and then downloaded the Microsoft subscription-only applications from the Mac App Store, you will not be able to activate them as you need a Microsoft 365 license. If you did this, uninstall the Microsoft 365 apps and reinstall Office 2019 for Mac and apply that license key.

You posted in a macOS High Sierra community with a profile that shows that you have macOS Big Sur installed. Where you post is important to avoid confusion and potentially the wrong answer.

Maybe the version of Microsoft Office is not compatible with your version of operating system or model/year computer. But this is only a guess until you provide some details and context.