First, you need to import css:
import "/node_modules/flag-icons/css/flag-icons.min.css";or use CDN:
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/redek-dp/flagicons@main/css/flag-icons.css" /><link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/redek-dp/flagicons@main/css/flag-icons.min.css" /><span class="fi fi-gr"></span> <span class="fi fi-gr fis"></span>