Hi Manh,
Thank you for reaching out. Make sure the client customer ID you are using is the ID of a client account and not a manager account. You will get a blank response if it is the ID of a manager account. If this is not the issue, please share the complete detailed request and response logs
via Reply privately to author so that I can further investigate the issue. You can find information on enabling logging
here.
Regards,
Mitchell
Google Ads API Team

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