Make sure you are entering the code completely and accurately. All registration codes are four capital letters, a hyphen, then four capital letters, e.g., XXXX-XXXX.
Once you’ve registered an account, you do not need to register again. When returning to the site, use the email address and password you set up during registration.
Did you purchase a used book? If so, the code may have been used already. You need to purchase a new code online.