Hi Juv,
Thank you for reaching out. 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 SOAP 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?
Thanks and regards,
Xiaoming, Google Ads API Team