Win 10 license
From C-net's website
"It should be noted that if you have a Windows 7 or 8 Home license, you can only update to Windows 10 Home..." perhaps you installed pro on a home license?
Also in past windows 95 thru win7 you couldn't use a factory install CD with a non factory CD key and visa versa. I know Win 10 is a different type install now so who knows. You might have to brake down and shell out the $139 to MS. OR better yet get a new PC, old windows 8.1 machine is probably on the slow side now! during the sales a new machine can be has for a few hundred dollars and is probably faster.