When you activate the license it sends back information that identifies the CPU, motherboard etc. Each component is weighted with points and if so many points change then you need a new license. The license number on your disk has probably been registered. I picked up a refurbished PC from Newegg once that was fine except the license number had already been registered on a different machine. Microsoft's advice was return it to Newegg.
That also happened to my brother-in-law a few months ago. He ended up purchasing a new copy of Windows 10 instead of upgrading off the install.