・ You entered wrong product key.
Check the entered product key whether it isn’t wrong(28 digits alphanumeric characters without hyphen).
Pay attention especially to similar alphanumeric characters. For example number “0” and english character “o”, number “8” and english character “B”, capital letter “I”, small letter “l” and number “1”.

・ Enter with half-width characters.
If you entered the product key with full-width characters, enter it with half-width characters.

・ Use another browser.
Depends on the browser, you may not enter the product key correctly.
In this case, use the another browser and resister again.
*If you aren’t using Google Chrome, resister with Google Chrome.