problem with office 2021 pro key

This is the product you purchased? https://www.gamers-outlet.net/en/microsoft-office-2021-professional-plus-iso

The price alone is a major red flag.

The Pro Plus product is not designed for home users. It is a volume license, which means it is intended for organizations, not individuals. These licenses are managed by an organization’s IT department and are tied to a contract with Microsoft. Because of how volume licensing works, activation may depend on things like installation limits or periodic validation through an organization’s KMS server. Those processes require IT support from the organization that owns the license. That is why these licenses are not meant to be sold to individuals. Home users typically do not have access to the IT infrastructure needed to manage or maintain them. Unfortunately, some third-party sellers offer these licenses at very low prices. They may seem like a good deal, but they are often resold in ways that violate the licensing terms. Most buyers are not aware of these details until problems occur. You've been scammed. If I were you, I would return it and demand your money back.

Try signing in with the Microsoft account that originally activated Office. If it still says the key is already used, contact Microsoft Support and ask them to reactivate or transfer the license after the hard drive replacement