Error: Client is unauthorized to retrieve access tokens (ServiceAccountCredentials) nodejs - google-api

Question

I have been trying to create an app to list all gmail labels of given user by using service account.
Service account have domain-wide delegation and it's raise error when I ran this script "Client is unauthorized to retrieve access tokens".
var {google} = require('googleapis');
const SCOPES = [
'https://www.googleapis.com/auth/gmail.readonly'
];
var emailToLoginWith = 'useremail#anydomain.com';
var key = require('json_key_file_name.json');
var jwtClient = new google.auth.JWT(
key.client_email,
null,
key.private_key,
SCOPES,
emailToLoginWith
);
jwtClient.authorize( function (err, tokens) {
if (err) {
console.log(err);
return;
}
console.log('tokens : ', tokens);
listLabels(jwtClient);
});
function listLabels(auth) {
var gmail = google.gmail({version: 'v1', auth: auth });
gmail.users.labels.list({
userId: 'user_id',
}, function(err, response) {
if (err) {
console.log('The API returned an error: ' + err);
return;
}
console.log('labels response', response);
var labels = response.labels;
if (labels.length == 0) {
console.log('No labels found.');
} else {
console.log('Labels:');
for (var i = 0; i < labels.length; i++) {
var label = labels[i];
console.log('- %s', label.name);
}
}
});
}

Answer 1

Client is unauthorized to retrieve access tokens.
This error normally happens when you have created the incorrect type of credentials on Google Developer console. You say that you are using a service account make sure that you have download the correct json file from. After that credentials and the code used to use them are diffrent. You cant use the credentials for a service account with code from browser you will get this error.
I am not a node developer but i found this
http://isd-soft.com/tech_blog/accessing-google-apis-using-service-account-node-js/
You may want to try to get the service account working with drive first. Then when that works move to gmail domain delegation with gmail and service accounts can be tricky sometimes its better to have an example you know works to build off of.