I have two questions.
1) what is the difference between EX407V27K and EX407K?
2) I am trying to attempt EX407, If I click "Enroll" button, Can't we unroll the exam? I am trying to understand, what will happen If I enroll
EX407K - It is a kiosk-based exam based on Ansible 2.3
EX407V27K - It is a kiosk-based exam as well but on Ansible 2.7
Regarding your second question.
If you want to enroll in the exam, then you need to schedule your exam in the Red Hat Exam scheduler.
If you want to unroll from the exam - I believe there is a way to manually unenroll from the exam. If you will get some trouble, you may reach the support for help.