Are you trying to type in a letter where it should be a number?
https://www.adobe.com/account.html for your Adobe orders page... or
Lost serial # http://helpx.adobe.com/x-productkb/global/find-serial-number.html
or
Online Chat Now button near the bottom for Activation and Deactivation problems may help
http://helpx.adobe.com/x-productkb/policy-pricing/activation-deactivation-products.html