问Heroku engines.node (package.json)未指定

EN

{
  "name": "my-app",
  "version": "0.1.0",
  "engine": {
    "node": "12.16.2",
    "npm": "6.13.4"
  },
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^4.2.4",
    "@testing-library/react": "^9.5.0",
    "@testing-library/user-event": "^7.2.1",
    "bcrypt": "^4.0.1",
    "body-parser": "^1.18.3",
    "cookie-parser": "^1.4.3",
    "express": "^4.17.1",
    "jsonwebtoken": "^8.5.1",
    "mongoose": "^5.4.20",
    "nodemon": "^2.0.2",
    "react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-scripts": "3.4.1"
  },
  "scripts": {
    "start": "node server/index.js",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject",
    "backend": "nodemon server/index.js"
  },
  "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"
    ]
  }
}

remote:        engines.node (package.json):  unspecified
remote:        engines.npm (package.json):   unspecified (use default)
remote:
remote:        Resolving node version 12.x...
remote:        Downloading and installing node 12.16.1...
remote:        Using default npm version: 6.13.4
<!-- more stuff -->
remote: -----> Build
remote:        Running build
remote:        
remote:        > my-app@0.1.0 build /tmp/build_016788c07037e47ee5638bf5c1b1f631
remote:        > react-scripts build
remote:
remote:        Could not find a required file.
remote:          Name: index.html
remote:          Searched in: /tmp/build_016788c07037e47ee5638bf5c1b1f631/public
remote: npm ERR! code ELIFECYCLE
remote: npm ERR! errno 1
remote: npm ERR! my-app@0.1.0 build: `react-scripts build`
remote: npm ERR! Exit status 1
remote: npm ERR!
remote: npm ERR! Failed at the my-app@0.1.0 build script.
remote: npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
remote:
remote: npm ERR! A complete log of this run can be found in:
remote: npm ERR!     /tmp/npmcache.yXjxR/_logs/2020-04-11T07_16_57_245Z-debug.log
remote: 
remote: -----> Build failed
remote:        
remote:        We're sorry this build is failing! You can troubleshoot common issues here:
remote:        https://devcenter.heroku.com/articles/troubleshooting-node-deploys
remote:
remote:        Some possible problems:
remote:
remote:        - Node version not specified in package.json
remote:          https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version
remote:
remote:        Love,
remote:        Heroku
remote:
remote:  !     Push rejected, failed to compile Node.js app.
remote:
remote:  !     Push failed