Hello Minh,
Thank you for contacting the Microsoft Community.
Allow me to provide assistance to address your concern for you to utilize your exam voucher.
In order for you to eliminate this error, make sure that you are signing in using your Personal Account. Sample of personal accounts are username@outlook.com or username@hotmail.com. Currently, Microsoft only accepts personal accounts when logging in to the Microsoft Certification profile.
Once you signed in, on the "exam discounts" page enter the email address where you receive communications about Microsoft Learn and the Cloud Skills Challenge (e.g. alias@company.com or alias@outlook.com) and continue to follow the steps provided on this Microsoft Build | Cloud Skills Challenge that you provided.
Let me know if additional assistance is needed.
If this solves your problem, please indicate “Yes” to the question and the thread will automatically be closed and locked. In case we do not receive a response, the thread will be closed and locked after one business day. Please be advised that after the case is locked, we will no longer be able to respond. If the resolution provided did not resolve the issue, please post a new question.
Regards,