Hello @Wonchul Park
Apologize for the inconvenience caused. Currently there is an portal bug for CDN which is not allowing you to provide source patter as "/ ".
As a temporary workaround, please use this link to update your configurations https://ms.portal.azure.com/?microsoft_azure_cdn=flight5.
Hope this was helpful. Please let us know in case of any additional questions or concerns.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.