"Delegation denied" in Google script

Question

I have this script:

function listLabels() {
  var response = Gmail.Users.Labels.list('contact@domain.com');
  if (response.labels.length == 0) {
    Logger.log("No labels found.");
  } else {
    Logger.log("Labels:");
    for (var i = 0; i < response.labels.length; i++) {
      var label = response.labels[i];
  Logger.log("- %s", label.name);
 }
  }
}

I got this error:

Delegation denied for user1@domain.com
where user1 - master account
contact - internal account created by user1
Grant access to your account - granted.

How can I fix this?