Sales Order Item custom field auto take value from item doctype and multiply it to the amount

ERPNext Selling/CRM
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1

I have added a custom field to the sales order item table in the sales order and want to fill this value with on its own by taking value from a custom field of item doctype the problem i am facing is whenever i try to push the value code the error message it gives that it doesn’t find the item even though item is saved in the sales order item table in the sales order doctype.

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2

this is the code i’m using to update tier rate but it is not showing any result

frappe.db.sql(“update tabSales Order Item SET tier_rate=%s where item_name=%s”,(CT,iname))