The hell?
The hell?
Reach out to Microsoft; they’ll handle it. They often face licensing problems (a colleague who works for them can assist with that).
I'm not claiming your rig is ready-made, but I'm stating the PRODUCT KEY you're using was designed specifically for a pre-built model. You'll need to purchase a different key for another product.
No, I purchased it through a PC store. Yes. Actually, I’ll give it another shot. All items came from a PC parts shop, even the operating system. If I need a new product key, I won’t waste time. I’ll just wait for Windows 10 to arrive.