Fetch sales order serial number to delivery note

I want to fetch value of field “against_sales_order” from Doctype “Delivery Note Item” in “Delivery Note”. I tried as below:
Step 1: add one custom field “item_link” in Delivery Note Form, and linked “Delivery Note Item”

Step 2: I add code below:
{% if doc.item_link %}
{% set u = frappe.get_doc(“Delivery Note Item”, doc.item_link) %}
{% endif %}

{{ u. against_sales_order }}`

But failed, please check the result:

Any one can give me some suggestion to achieve this function? URGENT!