i keep getting a 401 when connecting to mailgun

Question

I'm doing a course that does not allow mw to use libraries and npm so this is the https request I have crafted

helpers.mailgunSendEmail = (toEmail, toName, subject, message, callback) => {
      // validate parameters
      const emailRegex = /\S+@\S+\.\S+/;
      toEmail = typeof(toEmail) === 'string' && emailRegex.test(toEmail) ? toEmail.trim() : false;
      toName = typeof(toName) === 'string' && toName.trim().length > 2 ? toName.trim() : false;
      subject = typeof(subject) === 'string' && subject.trim().length > 2 ? subject.trim() : false;
      message = typeof(message) === 'string' && message.trim().length > 2 ? message.trim() : false;
      
    
      if (toEmail && toName && message) {
          // Configure the request payload
          const payload = {
              'from' : 'Pizza App <weixinpua0227@gmail.com>',
              'to' : "weixinpua0227@gmail.com",
              'subject' : subject,
              'text' : message
          };
    
          // Stringify the payload
          const stringPayload = querystring.stringify(payload);
          
          // Configure the request details
          const requestDetails = {
            'protocol' : 'https:',
            'hostname' : 'api.mailgun.net',
            'method' : 'POST',
            'auth' : config.mailgun.mailgunCredential,
            'path' : '/v3/' +config.mailgun.sandbox+ '/messages',
            'headers' : {
                'Content-type' : 'application/x-www-form-urlencoded',
                'Content-length' : Buffer.byteLength(stringPayload)
            }
            };
    
            // Instantiate the request object
            let req = https.request(requestDetails, res => {
                // Grab the status of the sent request
                let status = res.statusCode;
                
                // Callback successfully if the request went through
                if (status == 200 || status == 201) {
                    callback(false);
                } else {
                    callback(status);
                }
            });

this is how I call it

helpers.mailgunSendEmail("wxpua27@hotmail.com",'weixin','no subject','msg',(status)=>{
  console.log(status)
});

this is where the sandbox domain is stored

  mailgun:{
    sandbox:"sandboxa3a26706f4b74f75b015e91df2f31ebb.mailgun.org",
    mailgunCredential:"key-ba7409a41e533fcaecc0454cdf93475b"
  }  

I hardcoded my email(weixinpua0227@gmail.com) into the code due to mailgun's sandbox only allowing mail to be sent to authorised user

my sandbox is also in US area so the hostname (api.mailgun.net) should be fine