React blank page or 404 page after npm run build, Where did the problem come from?

I post question about index.html showing empty page, after npm run build(react).

I've done a lot of googling, but I don't think I found the right answer. Can you find a solution that fits my situation?

First of all, I tried this.

1. adding in package.json

homepage : "./" or "." or "absolute file PATH"

And my package.json is...

{
  "name": "client",
  "homepage": "./",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^5.14.1",
    "@testing-library/react": "^11.2.7",
    "@testing-library/user-event": "^12.8.3",
    "axios": "^0.21.4",
    "bootstrap": "^5.1.0",
    "dotenv": "^10.0.0",
    "html-react-parser": "^1.3.0",
    "http-proxy-middleware": "^2.0.1",
    "quill": "^1.3.7",
    "react": "^17.0.2",
    "react-bootstrap": "^2.0.0-beta.6",
    "react-dom": "^17.0.2",
    "react-ga": "^3.3.0",
    "react-html-parser": "^2.0.2",
    "react-icons": "^4.2.0",
    "react-kakao-maps-sdk": "^1.0.3",
    "react-loading-skeleton": "^3.0.1",
    "react-quill": "^1.3.5",
    "react-redux": "^7.2.5",
    "react-router-dom": "^5.3.0",
    "react-scripts": "4.0.3",
    "react-use-clipboard": "^1.0.7",
    "redux": "^4.1.1",
    "redux-devtools-extension": "^2.13.9",
    "redux-persist": "^6.0.0",
    "redux-promise": "^0.6.0",
    "redux-thunk": "^2.3.0",
    "use-react-router": "^1.0.7",
    "web-vitals": "^1.1.2"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "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"
    ]
  }
}

and my app.js file is...

import React, { useEffect } from "react";
import { Switch, Route, BrowserRouter } from "react-router-dom";

//import some components omitted

const App = () => {
// some logical code
return (
        <BrowserRouter history={history}>

            <Switch>
                <Route path="/contact" component={Contact} exact />
                <Route path="/withdrawal" component={Withdrawal} exact />
                
                {* and some Route code omitted *}

                <Route path="*" component={ErrPage} />
            </Switch>
        </BrowserRouter>
    );
};

export default App;

What's the problem with my code? Or what setting should I do more?

Thank you in advance.