Hi Woo-Keun Song,
Thanks for reaching out to Microsoft Q&A.
It may be to protect you from incurring for charges beyond your free/credit assigned limit. Did you try to upgrade your subscription to see if that works? It should let use freely post that action.
See all the available states of the subscription below.
https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/subscription-states
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