If you have installed office through Microsoft HUP, then you have to enter the product key.
Check the method of entering office product key here.
Install Office 2016/2013 : office.com/setup
Get Microsoft HUP Product Key:
Go to microsofthup.com.
- Select the Order History link in the upper right-hand corner of the page.
- Enter the work email address that you used to buy Office. This will send an email to yourself that contains a link to your order history.
- In the email you receive, select the link to view your order history.
- On the Order History web page, select your order number.
Your product key is listed under Microsoft Office Product Key (FPP) on the Order Details web page.
Reinstall Office using Microsoft HUP Product Key:
In the Sign in to set up Office window that appears when you first start an Office app, select I don’t want to sign in or create an account.
In the Enter office product key screen, enter the product key to activate Office.
Agree to the License terms by selecting Accept and start Word.
for more details, visit : www.office.com/setup