The ability to delete a given sale or purchase order depends of the state of the document. You can only delete 'Draft' orders. Otherwise you will be able to 'Cancel' the document or even sometimes you wont be able to change anything on the Order.
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Once Draft Orders are deleted, you'll see a message in at the head of the list displaying "Deleted."
This action can be performed one by one, using the same menu on each document you wish to Cancel.
Orders in the state of 'Confirmed', 'Offered' and 'Rejected' can't be cancelled. This is because those states are the final one in the workflow.