The problem was in action of form in my html
page:
<form action="{% url 'task_details' task.slug %}" method="POST">
{% csrf_token %}
{{ form.feedback_content }}
<div class="panel-buttons">
<div class="checkbox">
{{ form.is_solved }}
</div>
<div class="save-btn-container">
<button class="btn btn--pill btn--green" type="submit">Send</button>
</div>
</div>
</form>