1. Make sure you’ve entered the serial correctly. The letters PS are usually UPPERCASE, but the other letters found in the serial are lowercase.
2. The challenge number can be either 9 or 10 digits long. Make sure you have 9 or 10 digits, not more or less.
In rare circumstance the window displaying the challenge number can be too small to display all 9 digits: make sure your system font is the default, and the size s set to a 10 or less.