MongoDb Atlas: UnhandledPromiseRejectionWarning - MongooseTimeoutError - mongodb

Question

I am new in mongodb and currently following this article: https://medium.com/#beaucarnes/learn-the-mern-stack-by-building-an-exercise-tracker-mern-tutorial-59c13c1237a1
I am using mongoDB atlas as a course suggested and also whitelist my IP in it as many answers suggested.
File: .env
ATLAS_URI=mongodb+srv://sagar:<mypass>#cluster0-2gcdi.gcp.mongodb.net/test?retryWrites=true&w=majority
server.js
const express = require('express');
const cors = require('cors');
const mongoose = require('mongoose');
require('dotenv').config();
const app = express();
const port = process.env.PORT || 5000;
app.use(cors());
app.use(express.json());
const uri = process.env.ATLAS_URI;
mongoose.connect(uri, { useUnifiedTopology: true, useNewUrlParser: true, useCreateIndex: true }
);
const connection = mongoose.connection;
connection.once('open', () => {
console.log("MongoDB database connection established successfully");
})
const usersRouter = require('./routes/users');
app.use('/users', usersRouter);
app.listen(port, () => {
console.log(`Server is running on port: ${port}`);
});
user.model.js
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const userSchema = new Schema({
username: {
type: String,
required: true,
unique: true,
trim: true,
minlength: 3
},
}, {
timestamps: true,
});
const User = mongoose.model('User', userSchema);
module.exports = User;
routes:users.js
const router = require('express').Router();
let User = require('../models/user.model');
router.route('/').get((req, res) => {
User.find()
.then(users => res.json(users))
.catch(err => res.status(400).json('Error: ' + err));
});
router.route('/add').post((req, res) => {
const username = req.body.username;
const newUser = new User({username});
newUser.save()
.then(() => res.json('User added!'))
.catch(err => res.status(400).json('Error: ' + err));
});
module.exports = router;
currently its showing Error in console:
UnhandledPromiseRejectionWarning: MongooseTimeoutError: Server selection timed out after 30000 ms
at new MongooseTimeoutError (C:\Users\sagar\OneDrive\Desktop\nodejs\mern-exercise-tracker\backend\node_modules\mongoose\lib\error\timeout.js:22:11)
I am also testing my api in postman where its showing:
Could not get any response
There was an error connecting to http://localhost:5000/users/add.
Error of my screenshot: