React 18 shipped yesterday (March 29th). ReactDOM.render has been deprecated in React18 and currently issues a warning and runs in a compatible mode.

click here to see details.

To resolve you can either revert to a previous version of React or update your index.js file to align with the React 18 syntax.

index.js

import { StrictMode } from "react";
import { createRoot } from "react-dom/client";

import App from "./App";

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

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

Watch this video for detail: