No invoicing orders exist for contract account

Hi. Can anybody help me with this problem:

I am trying to generate an invoice from the system just using the t-code- FKKINV_s

then there is a message that is

"No invoicing orders exist for contract account 1004400/business partner 500000408

Message no. FKKINV115"

Diagnosis

For the contract account/business partner, and, if applicable, other selection criteria specified, no invoicing orders were selected that could be processed in invoicing process IN.

Can anyone explain the reason for (and possible solution) to the above?

Thanks in advance. Points will be awarded.

Prakash