Welcome to the forum.
First, a few questions, please:
What OS are you running?
What version of PrE?
What exactly happens, when you put in your S/N? Error message, and if so, exactly what does it say?
Please use the following link to reach Adobe Chat with your serial number issue. You should get immediate and effective resolution to your issue. After the link opens, click on "I still need more help." and the Adobe Chat pop up will appear.
Please let us know the outcome.