Hello, thank you for reaching out to Microsoft Q&A.
Try redeeming your Product Key here.
https://setup.office.com/redeem/enter-key
Copy and paste the link in an incognito tab.
Click Get Started.
Enter your product key.
Before you redeem your product key, double-check that you are using the correct email address.
The product will be assigned to your Microsoft account - https://account.microsoft.com/services
Once done, you can simply restart Office applications or your PC and sign in with your account.