manual accruals are not llowed for this agreement type

Hi,

I am facing a error while doing rebate settlement.i have maintained customer rebate agreement.created sales order,delivery and billing then updated in SE38.Now while doing settlement in VBO2, i am getting an error as "manual accruals are not allowed for this agreement type".May be billing document is not updated or what?maximum amt field is also coming as zero, which i think should be come as rebate value.Plz guide me.