"React must be in scope when using JSX" (react/react-in-jsx-scope with "window.React = React" on index.js)

Import React on top of your Menu.js file:

import React from 'react'

React should always be imported in a particular file, that uses JSX if you are working with this library (React) in your project.

This happens due to “React” import necessary in JSX file. The React library must also always be in scope from JSX code because JSX compiles as a react.

in your case 'React' must be import in Menu.js,

import React from "react"; 

this is an error most beginner react developers made.

And also You can refer