Error while creating Resource Related Billing

Hi, I've created service notification from IW51 and then service order for the same. I've created confirmation for the order from IW41. Now when I'm attempting to create resource related billing document in DP90 referring the service order, the system is showing "No expenditure items found".

I want to know the cause for this problem and the procedure to remove. the same.

Regards,

Pavi Ranjan