[A] It may so happen that if you try to cancel a document, the cancel option might not be visible. For example in the purchase order given below, the cancel option is not available.
[B] This is because there must be other documents created in that transaction after creating this document, in case of invoice you must have logged payment against it.
[C] In order to cancel the document you will have to cancel all the entries made after creating that document.
Keywords - Can not cancel OC, cancel option is not showing in invoice, cancel inward, cancel PO