postman returning 404 for the node.js endpoint

I am trying to create a simple rest endpoint using node.js. I am following the tutorial https://medium.freecodecamp.com/building-a-simple-node-js-api-in-under-30-minutes-a07ea9e390d2

I am able to run the command npm run dev, and the output shown is:

> notable@1.0.0 dev /Users/tejanandamuri/Desktop/notable
> nodemon server.js

[nodemon] 1.11.0
[nodemon] to restart at any time, enter `rs`
[nodemon] watching: *.*
[nodemon] starting `node server.js`
We are live on 8000

after this. in the postman, when I try to call the http://localhost:8000/notes, it is returning 404 error with the response body Cannot POST /notes

Here are my files:

server.js:

const express        = require('express');
const MongoClient    = require('mongodb').MongoClient;
const bodyParser     = require('body-parser');
const app            = express();
const port = 8000;
require('./app/routes')(app, {});
app.listen(port, () => {
  console.log('We are live on ' + port);
});

index.js:

// routes/index.js
const noteRoutes = require('./note_routes');
module.exports = function(app, db) {
  noteRoutes(app, db);
  // Other route groups could go here, in the future
};

note_routes.js

module.exports = function(app, db) {
  app.post('/notes', (req, res) => {
    // You'll create your note here.
    res.send('Hello')
  });
};

via Teja Nandamuri