If you see the message "This product key has already been used," this means that you've already redeemed your product key and added your Office product to your Microsoft account. Don't worry, this message doesn't mean you can't install or activate Office.
Use your Microsoft account to install and activate Office
Use your Microsoft account, not your product key, to install and activate Microsoft 365, or Office 2016 and individually purchased Office apps, such as Project, Visio, Word, Excel, and Outlook.
Sign in to install Office
Bookmark the site below so you always know where to go to install Office.
Sign in to activate Office
If an Office app prompts you to activate, sign in with your Microsoft account instead of a product key.
Use the right Microsoft account
Make sure you sign in with the Microsoft account that you used to buy Office.
Note
There's no need to go to Office.com/setup to install or reinstall Office later. This site is only used for redeeming new, unused product keys after purchase. After your product key is redeemed, your Office product is added to your Microsoft account and you don't need a product key to install or activate Office.