我正在尝试按照教程来启动我的第一个NodeJS应用程序，但是我不明白为什么会出现此错误。我正在尝试将MongoDB数据库与该应用程序连接，并且显然必须修改我的Start.js文件才能执行此操作……但是错误表明我完全丢失了该文件。这是我的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}`);
});

有人告诉我也创建一个.env文件来放置在项目文件夹中，这可能与它有关吗？也许数据库的地址不正确？任何帮助将不胜感激。

编辑：这是我的package.json文件

{
  "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"
  }
}