When you first start the setup, you need to enter a default installation key which is not really a key you can activate, it is simply a key used for installation sake. then when you are in Windows, you can use your real product key to activate it.
Google it, default installation keys, there are a few depending on which flavor of Windows 8.1 you are installing (ie. Windows 8.1 Core or Windows 8.1 Pro or Windows 8.1 Single Language, etc.)