В вашем стартовом скрипте есть лишняя запятая (,).
Вот как это должно быть:
package.json:
{
"name": "reactapp",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"start": "webpack-dev-server --hot"
},
"author": "safuwan",
"license": "ISC",
"dependencies": {
"babel-core": "^6.26.3",
"babel-loader": "^7.1.4",
"babel-preset-es2015": "^6.24.1",
"babel-preset-react": "^6.24.1",
"react": "^16.4.1",
"react-dom": "^16.4.1",
"webpack": "^4.14.0",
"webpack-dev-server": "^3.1.4"
}
}