Hi,
Thank you for reaching out. You would need to set the login-customer-id if your access to the customer account is through a manager account. This header is required and must be set to the customer ID of the manager account. To check if the client is under a MCC, in Google Ads UI, you need to go to Tools > Account Access(Under setup) > Manager tab. However, if you are using a root MCC, it shouldn't be under any manager.
Let me know if this helps.
Regards,
Dave
Google Ads API Team