You should be giving a single component to element property. Assuming that you wanna Header on top of every route, try with this :

import './App.css';
import Header from './Header';
import Home from './Home';
import { BrowserRouter as  Router, Routes, Link } from "react-router-dom";

function App() {
  return (
    //BEM
    <Router>
        <div className="app">
          <Header />
          <Routes>
             <Route path="/" element={Home} />
          </Routes>
        </div>
    </Router>
  );
}

export default App;

If you want to have Header only on path /, a cleaner way is to put it inside Home. Otherwise you could do it like so:

import './App.css';
import Header from './Header';
import Home from './Home';
import { BrowserRouter as  Router, Routes, Link } from "react-router-dom";

function App() {
  return (
    //BEM
    <Router>
        <div className="app">
          <Routes>
             <Route path="/" element={()=><><Header/><Home/></>} />
          </Routes>
        </div>
    </Router>
  );
}

export default App;

Try to wrap multiple elements in Fragment

<Route path="/" element={<><Header/><Home/></>}/>

Have you tried wrapping them in fragment?

<Route path="/" element={<><Header/><Home/></>}/>