That error simply means you typed the Serial wrong.
If you think to have inserted it correctly, the most likely mistake you made is that, if you are doing a web activation (because a firewall or antivirus is blocking access from the app to the activation server), be sure you don't confuse the "Installation id", which is asked by the Web page and you DO NOT have to enter, since it's already pre-filled, with the Serial Number.
In the "Register Serial" you must insert the Serial Number, exactly identical to your purchase receipt, including hyphens and respecting upper/lower case, and not confusing the "0" (zero) with the letter O, the number 5 with the letter S. It must be identical.
If you are doing the web-based activation, the "Install ID" shouldn't be touched: it's already pre-filled, and you only need to press the "Submit" button, to get the activation key back.