Hi Kian,

You should note the below points before activating Windows using license key.

• A Windows 7 or Windows 8/8.1 product key cannot activate Windows 10. Windows 10 uses its own unique product key.
• By default, Windows 10 builds are pre-keyed, meaning, you do not have to enter a product key and should not be prompted to enter one, as long as you upgraded from an activated Windows 7 or Windows 8 or Windows 8.1 license or Windows 10 Preview build.
• If you perform a clean install on the first attempt, you will have to start over from an activated Windows 7 or Windows 8.x installation.
• If you are prompted to enter a product key, click Do this later and give it a few days, Windows 10 will activate automatically.

Product Activation in Windows 10
In previous Windows releases, when you installed an upgrade version of Windows: if you wanted to reinstall the upgrade version, you had first to reinstall the qualifying version from which you upgraded, reactivate it, then upgrade again and reactivate again.
With Windows 10, this is no longer the case. Once you are upgraded to Windows 10 and activated, you can simply create a bootable copy on DVD or USB and reinstall just Windows 10 without the need to reinstall the qualifying version, and it will reactivate automatically. Windows 10 registers the device with the Product Activation Center and keeps a copy of your device’s activation in a Cloud Store. When you perform reinstall or clean install the installation checks with Activation Server, detects the unique installation ID and produces the validation for your device.

When Installing Windows 10 Build 10240
To activate Windows 10 build 10240 (RTM), you must upgrade from a previous qualifying operating system that is genuine and activated. This includes Windows 7, Windows 8/8.1, and Windows 10 Insider Previews.
If you are running the Windows 10 Enterprise Preview, you either need to stay in the Windows 10 Insider Preview Program, or activate using a product key from the Volume License Service Center (VLSC.) If you are not eligible to be running Windows 10 Enterprise, I suggest you make preparations to roll back to the original version of Windows your system was licensed for, and make a reservation to upgrade to Windows 10.

Go to Settings (press Windows key + I) > Update & security > Activation.

I hope this helps.

Regards,
Lokesh