Warranty service order

Hi,

I have a warranty service order (copy from SM01) scenario. I know I cant create any revenue in this kind of scenario. I am trying to link my sales order ( debit memo request to my service order). I am getting the error message as:-

"No cost management is provided for sales document item 2050000842 000010

Message no. IX057"

Please advise me on this

Regrds

Ashish