Some Office products pre-installed on Windows devices can be activated without a product key, and some of those Office products do not require a Microsoft account during activation. If your Office product doesn't include a product key and can be activated without your Microsoft account, you must reinstall Office from the Microsoft Store app.
Reinstall Office 2021 from the Microsoft Store Library
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Select Start and then open the Microsoft Store app on your Windows device.
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From the Microsoft Store app, select Library icon > Apps tab (If you can see All tab in Library, select All tab.)
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Turn off the Show installed products only filter under Sort and filter (If you select All tab, skip this step.)
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From Apps (or All) tab, find the Office product, and select Cloud button.
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Office installer will be downloaded and then Office installation will start.
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Follow the prompts to complete the installation.