If you have product key then you can enter product key on office setup.
Go to www.office.com/setup.
Then, sign in with the Microsoft account that you used to buy Office.
Instead, use your Microsoft account to sign in to microsoft office.
Then install your Office product.
This means your product key has already been redeemed and is no longer needed.
After you sign in, select your name and then select Order History.
Locate individual Office app.
Select Install Office to view your 25-character code.
Select Install Office again in the window where the product key appears.
Let's get started page.
Follow the prompts to sign in again and associate your product with your Microsoft account.