Third device in my family subscription keeps asking for Product Key

Microsoft 365 Family does not use a product key for a family member’s device after sharing. The product key is only for the initial redemption. For the niece’s laptop, Office should be activated by signing in with the Microsoft account that accepted the family sharing invitation.

Try this:

1. Confirm the sharing is active.
   • The subscription owner can verify that the niece is included in the family group and that sharing is active.
   • Sharing can be used with up to five other people in the family group.
2. On the niece’s laptop, sign in with the correct Microsoft account.
   • Open Word, Excel, or PowerPoint.
   • Go to File > Account.
   • Sign out of any accounts shown there.
   • Close all Office apps.
   • Reopen an app and sign in with the Microsoft account that accepted the shared Microsoft 365 Family invitation.
   • Expected result: Office should show the subscription through the shared Family plan instead of asking for a product key.
3. If the product key prompt still appears, reinstall from the signed-in account.
   • On the niece’s laptop, sign in to the Microsoft account that accepted the invitation.
   • Go to the Microsoft account Services & subscriptions page and install Office from there.
   • Do not try to enter a product key again. A redeemed key cannot be reused or shared.
4. If needed, remove and re-add the sharing.
   • Remove the niece from the family sharing setup.
   • Send a new sharing invitation.
   • Have her accept it with the same Microsoft account she will use inside Office.
   • Then sign in again in the Office apps on her laptop.

Important details for this scenario:

• Seeing the niece’s laptop listed under devices means the subscription sharing is recognized, but Office activation on that laptop still depends on signing in with the correct Microsoft account.
• Do not share the product key with family members. Product keys cannot be used to share a Microsoft 365 Family subscription.
• If Office was preinstalled on the niece’s laptop, the app may show a trial or activation screen. In that case, signing in with the shared Microsoft account or reinstalling from the Services & subscriptions page usually resolves it.

References:

• Using product keys with Microsoft 365 | Microsoft Support
• "This product key has already been used" error in Office | Microsoft Support
• Share your Microsoft 365 subscription | Microsoft Support
• Manage Microsoft 365 subscription sharing | Microsoft Support
• How do I find my product key - Microsoft Q&A
• Purchase successful but not able to associate with account. - Microsoft Q&A
• Where do I find product key for Microsoft 365 Family subscription? Family member prompted for product key when opening Excel. - Microsoft Q&A