Since 4 days this mysterie happend with my PS6 and Adobe cannot help. Its an impossible situation.
First sign in with your Adobe ID and Password. You do not have to buy anything, and you are not signing up for creative cloud.
You will then be asked to "license this product" or you will be asked for a serial number directly.
Select "I have a serial number", and enter your serial number.
Photoshop CS6 should then be licensed and working.
Gene