Do you by chance have another test taker account for another company's certification exams using Webassessor/Kryterion? Unfortunately, a limitation of Kryterion, our testing system, is that there cannot be two test taker accounts using the same email address in their system.
To fix this issue, please change one of your account email addresses to something else. When you login to your MuleSoft test taker account after this, you should be able to login without a problem.
Assistance regarding difficulties during the exam can be found in this article.