Failed to compile. Module not found: Can’t resolve ‘react-router-dom’

After npm start, the browser gives the error:

Failed to compile ./src/components/App/App.js Module not found: Can’t resolve ‘react-router-dom’.

React-router-dom has been added to the dependencies in npm and so has react-router and react.

Project has been created using the create-react-app myapp cmd line. This is runned on a localhost, node server. I have an api and app folder inside my project folder. I have tried various things. Updated manually my package.json inside the app folder, reinstalled react-router-dom, delete the package-lock.json in the app folder and reinitialize it. My api folder holds nothing but node_modules, my api file, route.js, config.js, index.js and also a package.json and package-lock.json. I have tried the npm build command in my app folder. It just creates a ‘build’ folder which holds the same files as my public folder inside my app folder. I also tried running yarn add react-router-dom.

//=========App.js file=========

//dependencies
import React, { Component } from 'react';
import { BrowserRouter as Router, Route } from 'react-router-dom';

//components
import Header from '../Header/Header';
import Footer from '../Footer/Footer';
import Home from '../Pages/Home';
import StudentOverview from '../Pages/StudentOverview';
import StudentDetails from '../Pages/StudentDetails';
import Management from '../Pages/Management';
import StudentAdd from '../Pages/StudentAdd';
import Exercise from '../Exercise/Exercise';

//includes
import '../../../public/css/kdg-fonts.css';
import '../../../public/css/normalize.css';
import '../../../public/css/responsive.css';
import '../../../public/css/say-my-name.css';
import '../../../public/css/style.css';

//Run
class App extends Component {
  render() {
    return (
      <Router>
        <div className="App">
          <Route path='*' component={Header} />
          <Route path='*' component={Footer} />
          <Route exact path='/' component={Home} />
          <Route exact path='/studenten' component={StudentOverview} />
          <Route exact path='/studenten/:id' component={StudentDetails} />
          <Route exact path='/beheer' component={Management} />
          <Route exact path='/beheer/add' component={StudentAdd} />
          <Route exact path='/oefenen' component={Exercise} />
        </div>
      </Router>
    );
  }
}

export default App;

//=========appfolder/package.json file=========

{
  "name": "saymyname",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "react": "^16.7.0",
    "react-dom": "^16.7.0",
    "react-scripts": "2.1.1"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "react-app"
  },
  "browserslist": [
    ">0.2%",
    "not dead",
    "not ie <= 11",
    "not op_mini all"
  ],
  "devDependencies": {
    "gulp": "^4.0.0",
    "gulp-changed": "^3.2.0",
    "gulp-clean-css": "^4.0.0",
    "gulp-rename": "^1.4.0",
    "gulp-sass": "^4.0.2",
    "gulp-uglify": "^3.0.1"
  }
}

UPDATE: npm install -S react-router-dom resolved the error.