This means that the sales order is linked to a sales invoice, which is preventing the deletion of the line...
If the sales order link is linked to a posted invoice, you will need to use the Maintenance Tool to revert the invoice back to outstanding in order to delete the line. If the invoice is outstanding, you will be able to remove the line immediately.
You will then be able to edit the quantity of the line or delete this by deleting into the delivery details tab. You will then need to re-create the sales invoice/ re-add the line to the outstanding invoice from the 'Available Items'
**Please note, you will need to delete the existing invoice from Sage to prevent duplicates.**