We have a manager account with google adwords which throws the following error AuthenticationError.USER_PERMISSION_DENIED.
As per documentation given on the adwords page link https://developers.google.com/adwords/api/docs/common-errors#AuthorizationError.USER_PERMISSION_DENIED
we are unable to figure out the solution that has to be taken in order to rectify the account.
Is re-authorizing the client secret key or resetting the developer token right option to address this exception?
If none of the above are the the viable solutions then let us know if any alternate way to resolve this issue.
ManagedCustomerService we were able to determine the accounts that have failed with USER_PERMISSION_DENIED error.this is the account ID that should match with your customer client id
this is the manager account ID that might be OR some other account id is in the field of your customer client id 
Sreelakshmi,