{{ purchase_order.products }}
is an array, not a single product item.
You need to loop through each product. Read about the available PO product variables in this guide.
Here’s a minimal example of it:
<table border="1" style="border-collapse:collapse">
<thead>
<!-- Table headings -->
<tr>
<th>Quantity</th>
<th>Price</th>
</tr>
</thead>
<tbody>
<!-- Loop over products -->
{% for product in purchase_order.products %}
<tr>
<td>{{ product.quantity }}</td>
<td>{{ product.price }}</td>
</tr>
{% endfor %}
</tbody>
</table>