Hi,
I work with Matt and allow me to assist you for now.
Creating campaigns under a MCC/manager (test or production) account is not possible. You will need to create campaigns under a specific client account.
That said, the clientCustomerId or customer_id you will need to specify in your API request header should belong to a client account, and not a MCC/manager account. I hope this helps.
Best regards,
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| Peter Laurence Napa Oliquino |
| Google Ads API Team |
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