To solve the react testing library error:

"ReactDOM.render is no longer supported in React 18, update the version of the react testing library."

Open your terminal in the root directory of your project and run the following commands:

npm install --save-dev @testing-library/react@latest
npm install --save-dev @testing-library/jest-dom@latest
npm install --save-dev @testing-library/user-event@latest

Make sure to update the versions of all react testing library packages you are using.

Your index.js file should use the new createRoot API to render your application.

index.js

import React from 'react';
import ReactDOM from "react-dom/client";

import App from './App';

const root = ReactDOM.createRoot(document.getElementById("root"));

root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

Now you should be able to start your tests without getting the error.

App.test.js

import {render, screen} from '@testing-library/react';
import App from './App';

test('renders react component', () => {
  render(<App />);
  const divElement = screen.getByText(/hello world/i);
  expect(divElement).toBeInTheDocument();
});