File is not included in React, what's wrong?

Question

React

Side.js and Side.css are in the comp folder
Inside Side.js I import Side.css styles but it gives an error

import React, { Component } from 'react'
import './Side.css'

export default class Side extends Component {
    render() {
        return (
            <div class="side df">
               Test 
            </div>
        )
    }
}