Remove a line item that was part of a purchase order upload

I'm very new to the DynamicsNAV world, I was able to upload purchase headers and lines, but i am not able to delete any of the lines on the purchase orders.  Receive this error message, and not sure what i did wrong.

 "Qty. Rcd. Not Invoiced must be equal to '0' in Purchase Line: Document Type=Order, Document No.=6587-2711_9756486, Line No.=90. Current value is '1'."