Getting success on dev mode but 404 on build.

[sahilpaudel]

package.json

{
  "name": "portfolio",
  "version": "1.0.0",
  "description": "",
  "keywords": [],
  "author": "Sahil Paudel",
  "license": "MIT",
  "private": true,
  "scripts": {
    "dev": "node server.js",
    "build": "next build",
    "start": "cross-env NODE_ENV=production node server.js",
    "export": "next export",
    "deploy": "rm -rf node_modules/.cache && next build && next export && touch out/.nojekyll && git add out/ && git commit -m \"Deploy portfolio to gh-pages\" && git subtree push --prefix out origin gh-pages"
  },
  "dependencies": {
    "@material-ui/core": "4.9.12",
    "@material-ui/icons": "4.9.1",
    "cross-env": "^7.0.2",
    "express": "^4.17.1",
    "next": "latest",
    "nprogress": "^0.2.0",
    "prop-types": "^15.7.2",
    "react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-ga": "^2.7.0"
  },
  "devDependencies": {
    "babel-plugin-transform-define": "^2.0.0",
    "gh-pages": "2.2.0"
  }
}

.babelrc.js

const env = require('./env-config')

module.exports = {
  presets: ['next/babel'],
  plugins: [['transform-define', env]],
}

next.config.js

const debug = process.env.NODE_ENV !== 'production'

module.exports = {
  exportPathMap: () => ({
    '/': { page: '/' },
    '/me': { page: '/me' },
    '/work': { page: '/work' },
    '/education': { page: '/education' },
    '/projects': { page: '/projects' }
  }),
  assetPrefix: !debug ? '/portfolio' : '',
}

env-config.js

const prod = process.env.NODE_ENV === 'production'

module.exports = {
  'process.env.BACKEND_URL': prod ? '/portfolio' : '',
}

Issue is I am getting 404 if I run yarn run build & yarn run start
but if i run yarn run dev it works fine.