Make sure you have the admin_order_status_notify email template
Do not use order_status_name but an order_status
with Allowed Placeholders: store_name, order_id, order_date_added, order_status, invoice, comment
Html Body: Order: {{ order_id }} <br/>
Date added: {{ order_date_added }} <br/>
Order status: {{ order_status }} <br/>
Invoice: {{ invoice }}<br/>
<br/>
Comment:<br/>
{{{ comment }}}<br/>
<br/>
Please reply to this email if you have any questions.