css였다...

client/src/index.js

@@ -2,7 +2,7 @@ import React from 'react';
 import ReactDOM from 'react-dom';
 import './scss/custom.scss';
 import 'bootstrap';
-import 'bootstrap/dist/js/popper.min.js';
+import 'bootstrap/dist/css/bootstrap.css';
 import App from './App';
 import reportWebVitals from './reportWebVitals';