It is due to OS
In your package.json, make sure to have your scripts(Where app.js is your main js file to be executed & NODE_ENV is declared in a .env file).Eg:
"scripts": {
"start": "node app.js",
"dev": "nodemon server.js",
"prod": "NODE_ENV=production & nodemon app.js"
}
For windows
Also set up your .env file variable having NODE_ENV=development
If your .env file is in a folder for eg.config folder make sure to specify in app.js(your main js file)
const dotenv = require('dotenv'); dotenv.config({ path: './config/config.env' });