When I scroll down on this page: https://academy.mendix.com/link/certifications/22/intermediate
I get 3 groupboxes which can be expanded. Two with registration options for the intermediate exam.
As these exams are online, you can register for any of the exams, as long as the time schedule fits your time zone.
Drop an email to firstname.lastname@example.org
Doesn't the page show the available exam sessions lower on the page? Or do you not have the option to scroll down?
Did you already pass the Rapid Certificate? I assume this course is only available once you've passed the Rapid exam.