It could be that there is no link between the manager account authenticated in the request with the client account specified in the headers or your OAuth2 credentials are from a user who does not have access to the client customer ID specified or any of its manager accounts.
This said, you should make sure that the OAuth2 credentials you are using were generated using an email address that has access to the account you are making the API request to. Also, you could use ManagedCustomerService.get() to determine which client accounts are linked to your manager account (see sample code for reference).
If error persists, could you provide the complete SOAP request and response logs that were generated when the error was encountered so I could investigate further? You may use the Reply privately to author option.