Kayıt olurken Sunucuya ulaşılamadı hatası React + MongoDB

const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');
require('dotenv').config();
mongoose.set('strictQuery', true);
const app = express();
// Middleware
app.use(cors({
  origin: ['http://localhost:3000', 'http://127.0.0.1:3000'],
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));
app.use(express.json());
// Connect to MongoDB
mongoose.connect(process.env.MONGODB_URI, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
})
.then(() => console.log('MongoDB connected...'))
.catch(err => console.log(err));
// Routes
app.use((req, res, next) => {
  console.log(`${new Date().toISOString()} - ${req.method} ${req.url}`);
  console.log('Request Body:', req.body);
  next();
});            
app.use('/api/users', require('./routes/users'));
app.use('/api/products', require('./routes/products'));
// Error handling middleware
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ message: 'Sunucu hatası', error: err.message });
});
const PORT = process.env.PORT || 5000;
app.listen(PORT, () => console.log(`Server running on port ${PORT}`));