From Add features FAQ's: Add features: FAQ - Windows Help
I suppose it could be that your country or region may be one that this isn't available in.You might not be able to buy a product key online in some countries and regions.
You can only add features to Windows 8.1, Windows 8.1 Pro, Windows 8.1 KN, and Windows 8.1 N, or Windows 8, Windows 8 Pro, Windows 8 KN, and Windows 8 N.
If Add features to Windows 8.1 or Add features to Windows 8 isn't available on your PC, you won't be able to install the Pro Pack or Media Center Pack.
Yep, that would be why then. It needs to be a retail or OEM license.