Sales Order No is not accepting when we doing down Payment through F-29

Hi All,

when we are doing customer down Payment thru F-29, system is not accepting sales order number in sales order feild itself.

if we enter quation or enqiry number system is accepting and posting, if we put sales order number feild empty also it accpting and posting, if we enter sales order number in sales order feild sytem is not accepting it is giving error message and saying entry xxxx does not exist in VBUK - check your entry

kindly help regarding this

thanks in advance

Kiran