You can only have one account with the same email addresss so you will receive an error if you try to Register with the email address of an old account.
When you complete the form and click the Register button and it still shows the form please scroll to the top of the form to see the "error message". In some cases it may appear that you got the "Security Code" wrong because the box is empty but this will NOT be the case if you do not see "Incorrect Security Code" in a red box in the area of the Security Code so check the error message that is displayed at the top of the form. For example it could be that the username you are using has already been used.
As noted in the "Blue Box" a new user account will require authorization by CADO before it can be used.
Please check your spam or junk mail folder for the verification email that is sent when you complete the registration if you do not see it in your inbox.
If you receive a registration error that states your email is already being used then please try the "Password Reset" to reset your password for that account.