Is it possible to use users.messages.insert() ? error could not handle the request

I am trying to execute the users.messages.insert() request as specified in the GMail API v1 (https://developers.google.com/gmail/api/v1/reference/users/messages/insert) but I get an error: could not handle the request...

Here is my script

`        function gMailInserMessage (sender_name, sender_email, msg_text) {

      // Create a new JWT client using the key file downloaded from the Google Developer Console
      const jwtClient = new google.auth.JWT(
        postoffice_key.client_email,
        null,
        postoffice_key.private_key,
        _.values(config_key.scopes.postoffice),
        config_key.admin_email // subject (or sub) impersonated user
      );

      return jwtClient.authorize().then(tokens => {
          // Obtain a new gmail client, making sure we pass along the auth client
          const gmail = google.gmail({
            version: 'v1',
            auth: jwtClient
           });

           // Base64-encode the mail and make it URL-safe 
           // (replace all "+" with "-" and all "/" with "_")
           var encodedMessage = btoa([
            'From: ' + sender_email + '\r\n',
            'To: ' + config_key.admin_email + '\r\n',
            'Subject: Contact\r\n\r\n',
            msg_text
          ].join("")).replace(/\+/g, '-').replace(/\//g, '_');

          //Make an authorized request to insert a User Messages
          return gmail.users.messages.insert({
            userId: config_key.admin_email,
            internalDateSource: 'receivedTime',
            neverMarkSpam: true,
            resource: {
              raw: encodedMessage
            }
          });
      }).then(res => res);
    }`

postoffice_key.client_email being the service account admin email address, this service accound has Domain-Wide Delegation Authority set ( users.message.send() is running fine...)

not an isseu ...

related to }).then(res => res);

should be

}).then(res => res.data);