Unable to CANCEL Purchase Orders

 After going to purchase order summary form and entering the .

PO number -> then by going to TOOLS->CONTROL and selecting CANCEL PO option--

an alert is coming once the document is cancelled it cannot be undone do u want to proceed ......

then after clicking ok --- here iam getting error message as :
please undo the changes to the document in the current revision before taking the cancel action. Cause: you are trying to cancel a document that has been altered since its last archived revision. Action: you must first undo the changes before you can cancel the document