symfony app css not working

{% block stylesheets %}
            {# 'app' must match the first argument to addEntry() in webpack.config.js #}
            {{ encore_entry_link_tags('app') }}

            <!-- Renders a link tag (if your module requires any CSS)
                 <link rel="stylesheet" href="/build/app.css"> -->
        {% endblock %}
      {% block javascripts %}
            {{ encore_entry_script_tags('app') }}

            <!-- Renders app.js & a webpack runtime.js file
                <script src="/build/runtime.js"></script>
                <script src="/build/app.js"></script> -->
        {% endblock %}