`<p>` elements can't be nested, so perhaps it is better to use the following:

      <h4>Contribution location</h4>
      {% if report.location %}
      <p>
        {{ report.location }}
      </p>
      {% else %}
      <p>
        Longitude: {{ report.longitude }}
        Latitude: {{ report.latitude }}
      </p>
      {% endif %}