<*** />is using incorrect casing. Use PascalCase for React components,

import React, { Component } from 'react';
import logo from './logo.svg';
import './App.css';
import toDoApp from './components/todoApp'

class App extends Component {
  render() {
    return (
      <div className="App">
        <header className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
           <toDoApp />
        </header>
      </div>
    );
  }
}

export default App;

上面是原始碼。沒看出有什麼問題。後來不斷測試。發現引入元件標籤第一位一定要大寫

import React, { Component } from 'react';
import logo from './logo.svg';
import './App.css';
import ToDoApp from './components/todoApp'

class App extends Component {
  render() {
    return (
      <div className="App">
        <header className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
           <ToDoApp
 
 />
        </header>
      </div>
    );
  }
}

export default App;