html block content printed as {% block content %} {% endblock %} string

Question

test.html

{% extends "main.html" %}
{% block content %}
<p>block test</p>
{% endblock %}

I add to main.html

{% block content %}{% endblock %}

main.html and test.html are in the same directory.

but in the main page literally {% block content %} {% endblock %} are printed like this ↓.

I don't know why. I've tried {% extends "./main.html" %} but also doesn't work.