Well, I suspect either you entered the key incorrectly or you were sold an invalid key.
Check for the usual mistakes, Letter "O" for number "0", "G" for "C", letter "B" and number "8", etc... Otherwise, if you know you're entering your key correctly, then it's likely an invalid key and you should attempt to get your money back.
It's for reasons such as this that we only recommend purchasing Microsoft Operating Systems from legitimate, authorized resellers.
-Wolf sends