I want to automate the process of Delivery Order creation once slae order is confirmed.

I tired the following:

I have add product serial (lot_id) in sale order line and once I confirm sale order, delivery order should be created with my product serial added across product(in split line on dc) and get validated automatically.

I tried calling these functions:

self.do_plus() #to add my populated serial number in split line lot_id
stock_operation = self.env['stock.pack.operation']
stock_operation.save() #to save the added serial id
stock_move = self.env['stock.move']
stock_move.assign_picking()
dc_validity = self.env['stock.picking']
dc_validity.action_done()

but ended up getting error as follows once I click confirm sale button,

You have a difference between the quantity on the operation and the quantities specified for the lots. 

Could you please let me know which functions I missed to call or if I am totally doing it the wrong way.