iI am trying to install Microsoft on my computer and it is telling me it was already installed by a user that is unknown to me

The message that the product key has already been used means the key was redeemed previously and is now permanently tied to a Microsoft account. Product keys can only be redeemed once and are not meant to be reused or shared.

To install and activate Office from a previously redeemed key, installation must be done by signing in with the Microsoft account that the key was originally associated with, not by re‑entering the key:

1. Go to the Microsoft account Services & subscriptions page: https://account.microsoft.com/services
2. Sign in with the Microsoft account that owns the Office license. If unsure which account was used, follow the guidance in Find the account you use with Office from the linked article.
3. Once signed in with the correct account, locate the Office product and select Install to download and install Office.
4. After installation, open an Office app and, if prompted, sign in with the same Microsoft account to activate. Do not enter the product key again.

If the product key is reported as already used by an unknown Gmail account, this indicates the key was redeemed to a different Microsoft account than the one being used now. Because Microsoft cannot move a redeemed license from one account to another, direct self‑service reassignment is not possible. In this situation:

• If the key was purchased from a reseller and appears to have been used or resold fraudulently, report the issue as suspected theft/fraud using the reporting path provided in the context.
• If help is needed identifying or accessing the correct Microsoft account, use the Find the account you use with Office and Get help with your Microsoft account support options, or contact a customer support agent from the Microsoft account help page.

Do not share the product key with others or attempt to redeem it again at Office.com/setup or Microsoft365.com/setup, as this will continue to show the “already been used” error.

References:

• "This product key has already been used" error in Office
• Get help with product key errors on Office.com/setup
• Why do I need an account to sign in to Microsoft 365 or Office?
• Get help with your Microsoft account
• Theft/Fraud Associated with my Office Product Key. - Microsoft Q&A