I'm attempting to deploy a NextJS app on my shared hosting server using the cPanel Setup Node.JS App section, but when I start the build - despite getting ready on http://localhost:3000 - the site throws a 503 error.

I've uploaded the build folder alongside the next.config.js, package-lock.json, package.json and server.js to the application root, and this is my current file structure:

next_main
    build (.next folder)
    node_modules
    next.config.js
    package-lock.json
    package.json
    server.js

This is my server.js file (exactly the same as what Next provided in their custom server docs):

const { createServer } = require("http");
const { parse } = require("url");
const next = require("next");

const dev = process.env.NODE_ENV !== "production";
const hostname = "localhost";
const port = 3000;

const app = next({ dev, hostname, port });
const handle = app.getRequestHandler();

app.prepare().then(() => {
    createServer(async (request, response) => {
        try{
            const parsedURL = parse(request.url, true);
            const { pathname, query } = parsedURL;

            switch(pathname){
                case "/a":
                case "/b":
                    await app.render(request, response, pathname, query);
                    break;
                default:
                    await handle(request, response, parsedURL);
            }
        } catch(error){
            console.error("Error occurred.", request.url, error);
            response.statusCode = 500;
            response.end("Internal server error.");
        }
    }).listen(port, error => {
        if(error) throw error;
        console.log(`> Ready on http://${hostname}:${port}`);
    });
}).catch(error => {
    if(error) throw error;
});

Failed to load next.config.js was also output in my stderr file, despite next.config.js being provided.

I've attached the current settings I have applied in my cPanel.

Please note that I do not have root access to the terminal, and am restricted to the next_main environment when running any NPM scripts.