MERN无法部署到Heroku

我正在尝试通过NodeJS渲染我的React应用程序并将其部署在Heroku上。但我似乎在浏览器中有此问题

未捕获到的SyntaxError：意外令牌'<'

该页是空白的白页。

我尝试了很多解决方案，但我似乎不知道哪里出了问题以及为什么首页不显示。

我在NodeJS中的Package.json

{
  "name": "portfolio-app",
  "version": "1.0.0",
  "description": "a simple app",
  "main": "app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "heroku-postbuild": "cd client && npm install && npm install --only=dev --no-shrinkwrap && npm run build",
    "start": "node app.js",
    "client": "cd client && npm run start",
    "dev": "concurrently -n 'server,client' -c 'red,green'  \"nodemon app.js\" \"npm run client\""
  },

  "dependencies": {
    "body-parser": "^1.19.0",
    "concurrently": "^5.2.0",
    "create-react-app": "^3.4.1",
    "express": "^4.17.1",
    "mongoose": "^5.9.6",
    "nodemon": "^2.0.2"
  },
  "engines": {
    "node": "10.x"
  }
}

我在React App中的package.json

{
  "name": "client",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^4.2.4",
    "@testing-library/react": "^9.5.0",
    "@testing-library/user-event": "^7.2.1",
    "history": "^4.10.1",
    "react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-scripts": "3.4.1"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "react-app"
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  },
  "proxy": "http://localhost:8080"
}

我的app.js服务器文件：

const express    = require('express'),
      app        = express(),
      bodyParser = require('body-parser'),
      nav        = require('./routes/navigation'),
      path       = require('path'),
      PORT       = process.env.PORT || 8080;

/* Settings */
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: true}));
app.use(express.static(__dirname + '/public'));



/*Production settings*/
if(process.env.NODE_ENV === "production"){
  app.use(express.static(path.join(__dirname, 'client/build')));

  app.get('*', (req, res) => {
    res.sendFile(path.join(__dirname, 'client/build', 'index.html'));
  });
}



/* Starting server */ 
app.listen(PORT, () => {
  console.log(`APP IS RUNNING ON PORT  ${PORT}`);
});