I have an error on my code, my frontend won't be able to access my backend, but when I only run my backend the code works perfectly.

ntent.js:25 AxiosError {message: 'Network Error', name: 'AxiosError', code: 'ERR_NETWORK', config: {…}, request: XMLHttpRequest, …}code: "ERR_NETWORK"config: {transitional: {…}, adapter: Array(2), transformRequest: Array(1), transformResponse: Array(1), timeout: 0, …}message: "Network Error"name: "AxiosError"request: XMLHttpRequest {onreadystatechange: null, readyState: 4, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, …}stack: "AxiosError: Network Error\n    at XMLHttpRequest.handleError (http://localhost:3000/static/js/bundle.js:41314:14)"[[Prototype]]: Error
(névtelen) @ Content.js:25
Promise.catch (aszinkron)
componentDidMount @ Content.js:23
invokeLayoutEffectMountInDEV @ react-dom.development.js:25133
invokeEffectsInDev @ react-dom.development.js:27351
commitDoubleInvokeEffectsInDEV @ react-dom.development.js:27327
commitRootImpl @ react-dom.development.js:26883
commitRoot @ react-dom.development.js:26682
finishConcurrentRender @ react-dom.development.js:25981
performConcurrentWorkOnRoot @ react-dom.development.js:25809
workLoop @ scheduler.development.js:266
flushWork @ scheduler.development.js:239
performWorkUntilDeadline @ scheduler.development.js:533
Content.js:36 XMLHttpRequest {onreadystatechange: null, readyState: 4, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, …}
(névtelen) @ Content.js:36
Promise.catch (aszinkron)
componentDidMount @ Content.js:23
invokeLayoutEffectMountInDEV @ react-dom.development.js:25133
invokeEffectsInDev @ react-dom.development.js:27351
commitDoubleInvokeEffectsInDEV @ react-dom.development.js:27327
commitRootImpl @ react-dom.development.js:26883
commitRoot @ react-dom.development.js:26682
finishConcurrentRender @ react-dom.development.js:25981
performConcurrentWorkOnRoot @ react-dom.development.js:25809
workLoop @ scheduler.development.js:266
flushWork @ scheduler.development.js:239
performWorkUntilDeadline @ scheduler.development.js:533
Content.js:41 {transitional: {…}, adapter: Array(2), transformRequest: Array(1), transformResponse: Array(1), timeout: 0, …}
(névtelen) @ Content.js:41
Promise.catch (aszinkron)
componentDidMount @ Content.js:23
invokeLayoutEffectMountInDEV @ react-dom.development.js:25133
invokeEffectsInDev @ react-dom.development.js:27351
commitDoubleInvokeEffectsInDEV @ react-dom.development.js:27327
commitRootImpl @ react-dom.development.js:26883
commitRoot @ react-dom.development.js:26682
finishConcurrentRender @ react-dom.development.js:25981
performConcurrentWorkOnRoot @ react-dom.development.js:25809
workLoop @ scheduler.development.js:266
flushWork @ scheduler.development.js:239
performWorkUntilDeadline @ scheduler.development.js:533
xhr.js:247     GET http://localhost:5000/Feladatok/ net::ERR_CONNECTION_REFUSED
dispat

import React, { Component } from 'react';
import axios from 'axios';

const Feladatok = props => (
  <tr>
    <td>{props.Feladatok.Kerdes}</td>
    <td>{props.Feladatok.Valasz1}</td>
    <td>{props.Feladatok.Valasz2}</td>
  </tr>
)

export default class FeladatokList extends Component {
  constructor(props) {
    super(props);

    this.state = {Feladatok: []};
  }

  componentDidMount() {
    axios.get('http://localhost:5000/Feladatok/')
      .then(response => {
        this.setState({ Feladatok: response.data })
      })
      .catch((error) => {
        console.log(error);
      })
  }

  feladatList() {
    return this.state.Feladatok.map(currentfeladat => {
      return <Feladatok Feladatok={currentfeladat} key={currentfeladat._id}/>;
    })
  }

  render() {
    return (
      <div>
        <h3>Logged feladatok</h3>
        <table className="table">
          <thead className="thead-light">
            <tr>
              <th>Username</th>
              <th>Description</th>
              <th>Duration</th>
              <th>Date</th>
              <th>Actions</th>
            </tr>
          </thead>
          <tbody>
            { this.feladatList() }
          </tbody>
        </table>
      </div>
    )
  }
}

const express = require('express');
const cors = require('cors');
const mongoose = require('mongoose');

require('dotenv').config();

const app = express();
const port = process.env.PORT;

app.use(cors());
app.use(express.json());

const uri = process.env.REACT_APP_MONGODB;
mongoose.connect(uri, { useNewUrlParser: true, useCreateIndex: true }
);
const connection = mongoose.connection;
connection.once('open', () => {
  console.log("MongoDB database connection established successfully");
})

const feladatokRouter = require('./routes/Feladatok');

app.use('/Feladatok', feladatokRouter);

app.listen(port, () => {
    console.log(`Server is running on port: ${port}`);
});

const router = require('express').Router();
let Feladatok = require('../models/Feladatok.model');

router.route('/').get((req, res) => {
  Feladatok.find()
    .then(Feladatok => {
      console.log(`Found ${Feladatok.length} documents`);
      res.json(Feladatok);
    })
    .catch(err => res.status(400).json('Error: ' + err));
});

module.exports = router;