Activation Key for Windows 8.1 on VirtualBox

I've just installed Windows 8.1 on my VirtualBox, using the ISO I've downloaded from:

https://www.microsoft.com/en-us/software-download/windows8

The problem was during the activation. I googled for the following product key:

334NH-RXG76-64THK-C7CKG-D3VPT

I entered the product key above and it's able to complete the installation. However, the key didn't work, returned the following error, and I was asked to type another product key:

Code: 0xC004C003

Description: The activation server determined that the specified product key has been blocked.

Why does this happen? Is there a way I can find an product key to activate Windows 8.1 on VirtualBox? I need it to run all the Windows 8.1 features on VirtualBox. Thanks for the advance!