[BelleNottelling]

invoice.lines is an array, so invoice.lines.lines is trying to reference an array key that does not exist.

You want something like so to iterate over each line item:

Code:

<ul>
  {% for line in invoice.lines %}
    <li>{{ line.title }} {{ line.price }}</li>
  {% endfor %}
</ul>

There is also line.period, line.quantity, line.unit, line.price, line.tax, line.taxed, line.charged, and line.total.