MailGun results in 401 forbidden using nodeJs

I am trying to send an email using mailgun and nodejs. I took the code MailGun provides you and added my domain name and api key:

const mailgun = require('mailgun-js');
const DOMAIN = 'mail.mywebsite.com';
const mg = mailgun({apiKey: "this-is-myApiKey", domain: DOMAIN});
const data = {
    from: 'Support <support@mywebsite.com>',
    to: 'myemail@gmail.com',
    subject: 'Hello',
    text: 'Testing some Mailgun awesomness!'
};
mg.messages().send(data, function (error, body) {
    console.log(1, error);
    console.log(2, body);
});

This results in a 401 forbidden, but if I change to my sandbox domain name, then it works. Does anyone have some helpful tips to fix this?

mail.mywebsite.com = domain name set under sending domains
this-is-myApiKey = my private api key found here: https://app.mailgun.com/app/account/security/api_keys