Thanks for reaching out. with regards to your concern, the error USER_PERMISSION_DENIED usually occurs when there is no link between the manager account authenticated in the request and the client account specified in the headers. Could you please make sure that the manager account you used to authenticate the call has administrative access to the client account you are calling against? If the issue persists, could you please send me the complete request and response along with the client customer id and the email you used to generate the OAuth2 credentials via Reply privately to the author option?
The 'NO_CUSTOMER_FOUND' error occurs when the customer/account specified in your request header is indeed not found or is not a valid account. Could you please verify if you are passing the correct client customer Id?
Thanks,
Nirmita Patel Google Ads Team