I got this from Academy:
Starting June 20th, we introduced new payment options for exam bookings. These changes will initially apply to Intermediate and Advanced exams, with updates for the Expert Certification to follow later. What is New?
You should be able to buy your certification. Hopefully an intermittent issue indeed. If issue persist, contact academy@mendix.com
Hi Haifeng,
This might be intermittent issue , can you try after sometime
It's probably your best best to reach out to Mendix Academy via academy@mendix.com to get support on this matter.