Why am I receiving the error: "Missing script: Start"? - node.js

Question

I'm trying to follow a tutorial for starting up my first NodeJS app, but I don't understand why I am getting this error. I'm attempting to connect a MongoDB database with the app, and obviously had to modify my Start.js file in order to do that... but the error says I'm missing that file completely. Here's my Start.js:
require('dotenv').config();
const mongoose = require('mongoose');
mongoose.connect(process.env.DATABASE, { useMongoClient: true });
mongoose.Promise = global.Promise;
mongoose.connection
.on('connected', () => {
console.log(`Mongoose connection open on ${process.env.DATABASE}`);
})
.on('error', (err) => {
console.log(`Connection error: ${err.message}`);
});
const app = require('./app');
const server = app.listen(3000, () => {
console.log(`Express is running on port ${server.address().port}`);
});
I was told to create a .env file to place in the project folder as well, could this have something to do with it? Maybe the address to the database is incorrect? Any assistance would be greatly appreciated.
EDIT: Here's my package.json file
{
"name": "demo-node-app",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"watch": "nodemon ./start.js"
},
"keywords": [],
"author": "",
"license": "ISC",
"dependencies": {
"body-parser": "^1.19.0",
"dotenv": "^8.2.0",
"express": "^4.17.1",
"express-validator": "^6.3.0",
"mongoose": "^5.8.3",
"pug": "^2.0.4"
},
"devDependencies": {
"nodemon": "^2.0.2"
}
}