Hi uyenllnh,
Thank you for contacting the Microsoft Community.
I am sorry to hear about the issue you are encountering when trying to use your voucher code. Allow me to further check and guide you with the process.
Please be advised that this error usually occurs if your student status is not yet validated. Note that you should see a green checkmark under the job title in your Certification profile. You do not need to verify that your email is eligible for the discount under the "Exam discount page" because it will appear on the check-out page.
To update your title to “Student”, please visit Identify yourself as a student in your certification profile. If you have already verified your student status and the error still persists, your account needs to be refreshed.
To do so, please follow the steps below using an InPrivate browser:
Login to Manage your certification profile using your Microsoft account and password.
Click “Edit profile”
Check your address and make sure to remove all special characters and diacritics
If there are no special characters, click “Submit” even without making any changes.
Once done, please re-login and try scheduling the exam again and check if the voucher code applied on the checkout page.
In the event that the steps didn’t work, please provide us with the step-by-step screenshot of the incident in a form of PSR (Problem Steps Recorder) starting from the sign in page until you reach the error along with the requested information in the private message.
I hope this helps.
I look forward to your response. Please note that if no response is received after two business days, the case will be closed and locked. After the case is locked, we will no longer be able to respond, even through Private Messages.
Regards,