Cannot insert the null into

When we are posting the receipt of PA Purchase Orders, there is an error is displaying as 

Cannot insert the value NULL to column PAUnpostedQty table 'MWO7.dbo.PA00501' column does not allow nulls.UPDATE fails. cannot insert the nulls into

 Afterwards we cannot see this entry under PROJECT Wise Report . So help me on this issue as soon as possible and awaited your reply at the earliest.

Best Regards

Thomas V Abraham