RE: "You cannot create a return order line for a service product that is not stocked."
Hi Saifullah W,
If you use a service product then you cannot create a return order because there is nothing to put back into stock.
What is it that you want to achieve with this return order?
Do you want to credit a customer for the warranty charges that you invoiced?
This can actually be achieved without a return order.
Just enter a new line in your SO with a negative qty/amount for the warranty charges and create a new invoice (credit note).
There is no need to go through the complete return order process if you want to create a credit note only.
Best regards,
Ludwig