Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi,
Was Microsoft Office previously activated on this device? If so, you may no longer need the product key. A product key can only be redeemed once, after which the license is permanently linked to the Microsoft account used for redemption. You can then use that account to activate Office.
If Office has not been activated yet, your best first step is to contact the seller. If the key is valid and unused, they may be able to verify it or work with Microsoft to help retrieve the key.