Hi @Dongjun Lee (이동준) Thank you for posting your question on Microsoft Q&A.
Not sure how you are trying to do it. Could you please try the below and let us know if you are still facing the same?
You can change the administrator role's password using the Azure portal.
Select your server in the Azure portal. In the Overview window select Reset password.
Enter a new password and confirm the password. The textbox will prompt you about password complexity requirements.
Select OK to save the new password.
If the above still do not help. Then I will suggest to open support ticket so we can have a closer look at this issue.
If you have a support plan you may file a support ticket, if you don't have a support plan. Let us know so we can unable a free one-time support ticket for you.
Regards,
Oury