Custom tariff number not available in purchase order item level.
Otherwise, you create a custom field and fetch the value, and then set it in print format.
You can also set like If you want to create a custom print format then custom tariff number get for apply like
{% for row in doc.items %}
Item Code: {{row.item_code}} <br>
Customs Tariff Number: {{frappe.db.get_value("Item",row.item_code,'customs_tariff_number')}}
{% endfor %}