Error on sendMail no refresh token is set

require ('dotenv').config();

const express = require("express")
const app = express()
const cors = require("cors")
const nodemailer = require('nodemailer');
const { google } = require('googleapis');

const CLIENT_ID = process.env.CLIENT_ID
const CLIENT_SECRET = process.env.CLIENT_SECRET
const REDIRECT_URI = process.env.REDIRECT_URI
const REFRESH_TOKEN = process.env.REFRESH_TOKEN

const oAuth2Client = new google.auth.OAuth2(
  CLIENT_ID,
  CLIENT_SECRET,
  REDIRECT_URI
);
oAuth2Client.setCredentials({ refresh_token: REFRESH_TOKEN });

// the below code didn't fix problem, hence commented
// oAuth2Client.refreshAccessToken(function(err, tokens){
//  console.log(tokens)
//  oAuth2Client.credentials = {access_token : tokens.access_token}
//  callback(oAuth2Client);

app.use(express.urlencoded({ extended: true }))
app.use(express.json())
app.use(cors())

app.get('/', cors(), async (req, res) =>{
    res.send("this is working")

})

app.post("/send_mail", cors(), async (req, res) => {
    res.send('ok')

  const output = `
    <p>You have a new contact request</p>
    <h3>Contact Details</h3>
    <ul>  
      <li>Name: ${req.body.name}</li>
      <li>Email: ${req.body.email}</li>
      
    </ul>
    <h3>Message</h3>
    <p>${req.body.message}</p>
  `;

async function sendMail() {
  try {
    const accessToken = await oAuth2Client.getAccessToken();

    const transport = nodemailer.createTransport({
      service: 'gmail',
      auth: {
        type: 'OAuth2',
        user: process.env.USERNAME,
        clientId: CLIENT_ID,
        clientSecret: CLIENT_SECRET,
        refreshToken: REFRESH_TOKEN,
        accessToken: accessToken
      },
    });

    const mailOptions = {
      from: '"Web name" <[email protected]>',
      to: '[email protected]',
      subject: 'Hello from gmail using API',
      text: 'Hello from gmail email using API',
      html: output
    };
    

    const result = await transport.sendMail(mailOptions);
    return result;
  } catch (error) {
    return error;
  }
}

sendMail()
  .then((result) => console.log('Email sent...', result))
  .catch((error) => console.log(error.message));

})

  app.listen(7000, () => console.log('Server started...'));