Hi,
To use AdWords API to target an account, you need log in as a user who has access to that account or its manager when you generate the refresh token. You'll not be able to use the refresh token generated from your manager account to target accounts that aren't linked to it. Please follow the steps provided
here to generate the refresh token for the required account.
For the second question, could you check that the clientCustomerId provided in the API request belongs to your AdWords client account and not your Manager account? If it is, could you tell me which service you are trying to use? If possible, please enable logging and provide the request and response logs so I can take a look. Please use Reply privately to author when responding.