as per we set form defaults for sale invoice there are comes default discount rate as per shown in belowe image.
but when we make add line in this line there are not coming default discount rate.
when we set in form defaults then it need to default for all lines which we are adding at the time of making invoice.
That is not how things work. The Form Defaults are applied as you see the form in the Settings tab. If the Sales Invoice Form Defaults page includes one line item with a discount, you will get a sales invoice with one line item with a discount.
If you want the default sales invoice to have multiple line items with discounts, you must enter multiple line items with discounts on the Form Defaults page.