If you haven’t installed Windows 10 on your PC before, you’ll have to perform an upgrade first before you can clean install.
If this doesn’t make any sense, that’s because Microsoft never makes
licensing easy, even when there’s a free version. Once you’ve performed
the upgrade and activated your Windows 10 upgrade on a PC, Microsoft
will associate your unique hardware ID with an activated Windows 10
license, and you can reinstall as you see fit. But not until you’ve
upgraded Windows 7 or 8 to Windows 10 and then activated.
Just keep in mind that Windows 10 is only provided for free as an
upgrade. You can’t clean install it, there are no keys at this point. Unless you use our clean install workaround.