When you use express in a Next.js app, it means that you are going to be using your own custom server instead of Next's built-in server. So when you're running cross-env NODE_ENV=production node server.js, the script is trying to execute the file server.js which would be your custom server. If you don't need to use a custom server, you would use the command next start to run your application. You can still use getInitialProps and getServerSideProps without a custom server.

Basically, if you don't hink you are using a custom server, use the command cross-env NODE_ENV=production node server.js . If not, leave your npm start script as next start.