React Bootstrap вам нужно импортировать
1.run command in cmd
npm install react-bootstrap bootstrap
2.after install add this to index.js
import 'bootstrap/dist/css/bootstrap.min.css';
//You may want to add individual components like 'Navbar',
// or 'Button' rather than the entire library.
3. add this, or similar to the js files that would be using the individual components
import {Navbar, Nav, Button} from 'react-bootstrap';
Shiny Shark