Please add in that you need to change some keys in app.js

[RoarGronmo]

The provided app.js from express doesn't quite fit the changes you are doing to the directory structure needed.

Plese add that you need to comment some of the command to succeed.

I.E: My origin app.js:

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public-flutter')));

app.use('/', indexRouter);
app.use('/users', usersRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

You may comment following lines to succeed.

//var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

//var indexRouter = require('./routes/index');
//var usersRouter = require('./routes/users');

var app = express();

// view engine setup
//app.set('views', path.join(__dirname, 'views'));
//app.set('view engine', 'jade');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public-flutter')));

//app.use('/', indexRouter);
//app.use('/users', usersRouter);

// catch 404 and forward to error handler
//app.use(function(req, res, next) {
//  next(createError(404));
//});

// error handler
//app.use(function(err, req, res, next) {
  // set locals, only providing error in development
//  res.locals.message = err.message;
//  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
//  res.status(err.status || 500);
//  res.render('error');
//});

module.exports = app;

[ozgur1aslan]

Worked like a charm! Thank you very much.