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This is happening because you have defined a variable named app_name in your application urls.py.
When you define that variable, it becomes your url namespace.
You either have to reverse your url like this:
<h1><a href="{% url 'polls:form_detail' pk=form.pk %}">
or remove the app_name variable to use the url name directly.
Read more about reversing url names @ django-docs
Source:stackexchange.com