@somoykhan/gatsby-theme-bootstrap-portfolio/src/pages/404.js

<div align="center">

import React from 'react'
import Seo from '../components/seo'
const NotFoundPage = () => {
  return (
    <div>
      <Seo title='Not Found' />
            page not found
    </div>
  )
}

export default NotFoundPage