Hi Koga,
Let me help you with your exam registration.
Indeed, AZ-302 is a transition exam intended for candidates who have passed exam 70-535.
Because of your described issue encountered,
I would recommend you following the troubleshooting guide below to eliminate issues upon accessing your account.
Please use Internet Explorer or Microsoft Edge and use an In-private browsing (Ctrl+Shift+P) as these are our recommended browser:
1. Open Internet Explorer (recommended browser)
2. Press 'Ctrl', 'Shift' and 'Delete' keys on your keyboard simultaneously
3. On the Delete Browsing History window, make sure to uncheck Preserve Favorites website data and check both Temporary Internet Files and Cookies then click Delete
If issues will still be encountered and for us to assist you further I have initiated a
private message. Included on this is the request is your profile information and screenshot of error message received, for me to locate/verify your profile and further investigate.
Also, for urgent exam registration you may contact the testing vendor Pearson VUE at:
Phone Number: 855-542-6924
Hours of operations: Mondays thru Fridays, 7 AM to 7 PM CT
Please note that student and Microsoft FTE discounts cannot be processed via phone registration.
To view your Private messages, please see the image below:

Looking forward to your response.
Regards,