Document is complete.
i have a document type Credit memo Cancel. when i try to post it manually to accounting, it gives me message "Document has already been Completed", whereas there is no Accounting document generated.
I suppose you have cancelled or closed the credit memo sales order itself , that is the reason you are getting the error.
Secondly also check wheather in VOFA wheather cancellation document type is assigned or not for the credit memo billing document type