Sorry, but you can’t activate this copy of Windows online. To fix this problem, you can either buy a new PC, or use a PC running Windows 7 and update to Windows 8.1 by following these steps (you'll need a blank DVD or USB flash drive):
On a PC running Windows 7, download the Windows 8.1 Upgrade Assistant. It will walk you through the steps to buy and download Windows 8.1.
When your copy of Windows 8.1 has finished downloading, tap or click Install by creating media and follow the instructions to burn a DVD or create a bootable USB flash drive.
On your Windows 8.1 PC, insert the DVD or USB flash drive, restart your PC, and follow the on–screen instructions to install Windows 8.1.
When prompted, enter the product key for your new copy of Windows 8.1. You can find it in your email receipt.