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#21
I'm not entering a Windows 10 home key, it's windows 7 ultimate.
Because the Windows 10 Home product key is not a retail key. It is the dummy "giveaway" key that everybody with the free Windows 10 upgrade has. It is blocked on Microsoft's activation servers just like OEM keys. If there were a way to enter a product key during the free upgrade you could do it that way, but there isn't. Contact the Store. When you buy online the digital entitlement is zapped to your computer. There will be no Pro product key sent you in an email. All you get is the download link in an email (I think).
As an aside: My wife installed Pro and then went online and bought Pro ($200) because she thought that was the way it worked. All she got was the digital entitlement for the already installed Pro. No download link. Microsoft sensed it was already istalled. (She didn't know about the Pro Upgrade (sigh).)
What you need to do is back up your files and restart with the Windows 10 media and reformat the hard drive. Then after you select Pro and installation proceeds put in the 7 Ultimate product key when asked. Or if you don't see a place to select an edition, putting in the product key when asked will make that selection. By the way, Pro is the default installation if no selections are made.