Hi John. What is the exact wording of the error message (omitting actual email addresses, for privacy)? Where are you seeing it? Is it in your email program? Or, if it's on WikiTree, what web page does it appear on? Knowing this information may help people to track down what is going wrong.
One possibility is that when he created his account, your cousin may not have completed a step required. He could look through his email, including spam, to see if there is a message with instructions from WikiTree which he has yet to carry out.