javascript – React js application is not starting to port 3000

I am trying to start the react application and by default it opens http://localhost:3000/portfolio and web page shows blank. and when I change the url to http://localhost:3000 from browser then it displays the content of the page. Why am I getting so? and How do I fix this ? This problem appeared when tried to deploy application using gh-pages.
Package.Json looks like:

   {
  "name": "portfolio",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^5.11.4",
    "@testing-library/react": "^11.1.0",
    "@testing-library/user-event": "^12.1.10",
    "framer-motion": "^4.1.17",
    "gh-pages": "^3.2.3",
    "normalize.css": "^8.0.1",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "react-particles-js": "^3.5.3",
    "react-router-dom": "^5.2.0",
    "react-scripts": "4.0.3",
    "styled-components": "^5.3.0",
    "tsparticles": "^1.31.1",
    "web-vitals": "^1.0.1"
  },
  "homepage": "https://username.github.io/portfolio",
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject",
    "predeploy": "yarn run build",
    "deploy": "gh-pages -d build"
  },
  "eslintConfig": {
    "extends": [
      "react-app",
      "react-app/jest"
    ]
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  }
}