Merhaba, react da index.js i icinde ki kodu header.jsx dosyasına aldım ama çalışmiyor neden olabilir.

index.js

//import { createRoot } from 'react-dom';
//import React from "react";
import ReactDOM from "react-dom/client";
import "./CSS/style.css";
import Header from "./header";

function  MainBody(){
    const whatWeWillLearn = "React";
    const lectureCount = 3;
  return(
          <div> 
          <p>This Course {whatWeWillLearn}<br></br>
          Total Lecture is {lectureCount}
          </p>
          
          <ul>
            <li>Call me</li>
            <li>welmar1k</li>
          </ul>
       {/*
           <div>
            Enter Task: <input maxLength={5} readOnly={false} placeholder="Yunus"></input>
  </div>  */}
  
          </div>);
}
function Students(){
const fullName = "Kris Walley";
const programmingExp = 2;
return(
      <div className="container p-4">
       <div className="row">Students Enrolled</div>
       <div className="row border">
           <div className="col-2">
              <img 
              src={'https://ui-avatars.com/api/?name=${fullName}'} 
              className="w-100"
              ></img>
           </div>
           <div className="col-10">
              {fullName}<br></br>
              Programming experience {programmingExp} years.
           </div>
       </div>
      </div>);
}
function Footer(){
  return(<p style={{color:"gray", backgroundColor:"black"}}>Happy Coding!</p>);
}
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
 <div>
  <Header/>
  <MainBody/>
  <Students/>
  <Footer/>
 </div>
  
 
 
);

header.jsx

function MainHeader(){
    return(<h1 className="heading1">REACT COURSE</h1>);
  }
  
  const subHeaderStyle ={
     color: "blueviolet",
     backgroundColor:"lightgray"
  }
  
  
  function SubHeader(){
    return(<p style={subHeaderStyle}>This will be an exciting course</p>);
  }
  
  function Header(){
  
    return(<div>
      <MainHeader></MainHeader>
      <SubHeader></SubHeader>
    </div>);
  }
  export default Header;