The Serial Number you get in the purchase receipt is never wrong.
Either you typed it wrong, or you are inserting the correct Serial number, but have selected another product for the activation, or you are inserting something else instead of the Serial Number (like the Order Number or the SKU number, none of them is the Serial Number, which is what the "Register Serial" function requires)
In any case, such kind of support requests are better handled by email, not on the forum, for the obvious reason that we can't discuss about your serial number on a public forum...