Look for the COA on the packaging for the original Microsoft software CD or DVD; typically it is on the back of the case or sleeve. Find the 25-character product key on the COA label.

The "Product Activation Failed" error in Microsoft Word and other Office applications (such as Excel, PowerPoint, etc.) ...

To fix the Microsoft 365 Apps activation error The account isn’t associated with this Office product, you can follow one of these methods.

Another free utility, Magical Jelly Bean Keyfinder serves up product keys for Windows and Office 2010 (it doesn't support more recent versions of Office) as well as non-Microsoft software.

Thus if you have purchased subscription-based version, you do not require the product key anymore since Office 365/Office 2021 subscriptions are linked to your Microsoft account.