Delete a Part Processed Sales Order

As soon as the Sales Order has been part processed, it cannot be deleted.
When you go into the Detail area on the Sales Order Batch screen, look for what is outstanding to Deliver and Outstanding to Invoice.
If detail lines have been delivered, they will need to be invoiced (so the Delivery Qty and Invoice Qty match)
As long as all lines for Delivered Qty and Invoice Qty match then you will be able to use the Close Order.