Why is the error in Django?

Question

Django

The pr_k variable contains the user id. I pass it to the template, but when I run django it throws an error:
TemplateSyntaxError at /messages/16/
Could not parse the remainder: '{{pr_k}}' from '{{pr_k}}'
Why?

<form action="{% url 'home:send_msg' pk={{pr_k}} %}" method="post">
      {% csrf_token %}
            {{form.post}}
      <br>
            <button type="submit" class="btn btn-default">Отправить</button>