Purchase Sales Soft Link Error

Dear Seniors,

We are trying to create a Purchase Order for few Items from the Sales Order screen the way we used to.  But right now, we are having an error which says "Purchase Sales Soft Link must be equal to ' ' in Sales Line: Document Type=Order, Document No.=SO100056, Line No.=40000. Current value is 'Item'.  We looked at the column for Purchase Sales Soft Link and saw that 3 out of 4 items have the word "Item" in it, the 4th item has blank.  Has anyone encounter this?

Thanks

BernaA