{% if article.metadata.parts_articles %}
<h4>Series</h4>
    <ul class="multi-parts-list">
        {% for part_article in article.metadata.parts_articles %}
            <li {% if part_article == article %}class="active-part"{% endif %}>
            {% if part_article != article %}
            <a href='{{ SITEURL }}/{{ part_article.url }}'>Part {{loop.index}}</a>
            {% else %}
            Part {{loop.index}}
            {% endif %}
            </li>
        {% endfor %}
    </ul>
{% endif %}

{% if article.metadata.parts_articles %}
<h4>Series</h4>
    <ul class="multi-parts-list">
        {% for part_article in article.metadata.parts_articles %}
            <li {% if part_article == article %}class="active-part"{% endif %}>
            {% if part_article != article %}
            <a href="{{ SITEURL }}/{{ part_article.url }}">Part {{loop.index}}</a>
            {% else %}
            Part {{loop.index}}
            {% endif %}
            </li>
        {% endfor %}
    </ul>
{% endif %}

Use double ("") quotation marks around HTML attributes
Use double ("") quotation marks around HTML attributes
