django.urls.exceptions.NoReverseMatch: Reverse for 'sign_up' not found. 'sign_up' is not a valid view function or pattern name

You have to fix the links in the template to target the website urls :

 {% if user.is_authenticated %}
        <li><a href="{% url 'website:logout' %}">Logout</a></li>
        {% else %}
         <li><a href="{% url 'website:login' %}">Login</a></li>
         <li><a href="{% url 'website:sign_up' %}">Signup</a></li>
 {% endif %}