New
#11
Windows 10 will only accept Windows 10 Product Keys (Retail Keys).
The upgrade process is activated with a general upgrade key that can't be used to activate your Windows 10 version. :) That's why with a clean install (after an upgrade from Win7/8.1), you need to skip the part to enter a product key, and after installation, Windows 10 will check with the Microsoft server if you already had an activated upgrade installed on that machine.