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.