When you are activating the software you will get this message if the serial number is not correct, if there are no activations left you would get a different message about the number of activations available.
You should try copying and pasting the serial number in again and ensure that you do not copy any characters from around the serial number, don't worry about the dashes.
If you get the serial in correctly you should see a green check mark indicating that it is correct. Let us know if you have any further trouble with this.