To be able to render content within the admin interface, first of all you need to extends from the HarmonyAdminBundle layout file, like:
{% extends '@HarmonyAdmin/default/layout.html.twig' %}
{% block content_title %}
{{ 'page-title'|trans }}
{% endblock %}
{% block main %}
{% endblock %}
{% block head_stylesheets %}
{{ parent() }}
{# ... #}
{% endblock %}
{% block body_javascript %}
{{ parent() }}
{# ... #}
{% endblock %}