This works :
{% with template_name=section.id|stringformat:"s"|add:".html" %}
{{ "./sections/"|add:template_name }}
{% endwith %}
outputting the right path which has the filename.
But this doesn’t :
{% include "./sections/"|add:template_name %}
Could not parse some characters: service|-orders/tabs/sections/"||add:template_name
I’m on Django 4.2