Error making a POST to mongoDB database

I am building a REST api service with Node, Express and MongoDB (A tutorial). I installed MongoDB and it runs normally on my PC on localhost:3000. In my server.js file I have this setup var express = require(‘express’); var mongoose = … I am getting this error as I post a POST command to mongoDB database. { “error”: “Validation Error” }. Please help.

Server.js

const express = require('express'); 

var mongoose = require('mongoose');

// Database connection and model.
require('./db/mongoose.js')();

const Book = require('./models/book.js');

// This creates our Express App.
const app = express(); 

// Define middleware.
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

// Listening on port 3000 (arbitrary).
// Not a TCP or UDP well-known port. 
// Does not require superuser privileges.
const PORT = 3000;

const bookData = {
  "book": {
    "title": "The Art of Computer Programming",
    "isbn": "ISBN-13: 978-0-201-89683-1",
    "author": { 
        "firstName": "Donald", 
        "lastName": "Knuth" 
    }, 
    "publishingDate": "July 17, 1997",
    "finishedReading": true
  }
}

const book = new Book(bookData);

// We will build our API here.
// HTTP POST /books
app.post('/books', async (req, res) => {
  try {
    const book = new Book(req.body.book);
    await book.save();
    return res.status(201).send({ book });
  } catch(e) {
    if(e instanceof mongoose.Error.ValidationError) {
      return res.status(400).send({ error: 'Validation Error' });
    } else {
      return res.status(500).send({ error: 'Internal Error' });
    }
  }
})

// HTTP GET /books/:id
app.get('/books/:id', (req, res) => {
    // ...
    console.log(`A GET Request was made! Getting book ${req.params.id}`);
});

// HTTP PATCH /books/:id
app.patch('/books/:id', (req, res) => {
    // ...
    console.log(`A PATCH Request was made! Updating book ${req.params.id}`);
});

// HTTP DELETE /books/:id
app.delete('/books/:id', (req, res) => {
    // ...
    console.log(`A DELETE Request was made! Deleting book ${req.params.id}`);
});

// Binding our application to port 3000.
app.listen(PORT, () => console.log(`Server is up on port ${PORT}.`));

mongoose.js

var mongoose = require('mongoose');

const MONGODB_URL = "mongodb+srv://tunji545:password@omisakinolatunji.chcjw.mongodb.net/dbname?retryWrites=true&w=majority";

module.exports = () => mongoose.connect(MONGODB_URL, {
  useNewUrlParser: true,
  useCreateIndex: true,
  useFindAndModify: false,
  useUnifiedTopology: true
});