Some users have reported issues with activating their copies of Windows 10, which could have been down to a number of different reasons. In some cases, the easiest way to get around the problem is to purchase a legitimate copy of Windows 10.
Unlike previous versions of Windows, the latest one is almost exclusively available online, which means that official Microsoft websites are the best bet for your purchase. The company was giving the OS away for free, but that promotion has now ended.
Microsoft has a helpful website that provides a downloadable copy in either 32-bit or 64-bit versions.
If you do find that you're unable to activate Windows 10 successfully, handily Microsoft introduced a new activation troubleshooter feature with the recent Anniversary Update. You'll find this in Settings, then click Update & Security, go to Activation and select Troubleshoot (you won't see this option if the OS has been activated).