ReactDom is undefined

There is a minor error in your code you have written ReactDom instead of ReactDOM. The correct code should be given below :

import React from 'react';
import ReactDOM from 'react-dom';
import Main from './components/main';

ReactDOM.render(<Main />, document.getElementById('root'));

You have ReactDOM but you don't have ReactDom(case sensitive)

ReactDOM.render(<Main />, document.getElementById('root'));

ReactDOM != ReactDom. There are two things you can do to fix this.

import React from 'react';
import ReactDOM from 'react-dom'; // you used 'react-dom' as 'ReactDOM'
import Main from './components/main';

ReactDom.render(<Main />, document.getElementById('root')); // you referred to it as 'ReactDom' which is wrong.

Becasue ReactDOM != ReactDom. So you need to fix one of those places so that both places have the same name with the same case.

The recommended fix would be,

import React from 'react';
import ReactDOM from 'react-dom'; // this is recommended 
import Main from './components/main';

ReactDOM.render(<Main />, document.getElementById('root'));

Or you can do,

import React from 'react';
import ReactDom from 'react-dom';
import Main from './components/main';

ReactDom.render(<Main />, document.getElementById('root'));