Why office setup product key required?
To install office you will require office setup product key.
There are steps to enter office setup product key on office.com/setup.
Before installing office setup first you need to activate office from office website so that you can manage your office subscription.
For both office setup offline and online ( office 365 ) its require to activate the setup.
After activation of office setup you can install setup file on your device.
follow steps for install microsoft office 2016, office 2010 and office 365
If you face any issue while installation contact for support else you need to activate your office setup again