Did you have Win 10 on PC before this? If so, it will activate based on the old install.
What you meant to do if fresh installing on a previous Win 10 PC is, when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and reactivate once finished. All the details of your PC are stored on a Microsoft server, PC checks it once win 10 installed to confirm your PC is on the list