"Order cannot be cancelled as some of the line items are already fulfilled" while trying to cancel the Order on Search Order screen.

As of now, System got designed in such a way that, once an Order got Packed and Invoiced system won't allow the user to cancel the Order. If the user still want to make the Invoiced inventory Live, then they have to re-GRN the item.

"Cannot mark order complete as it is not completely fulfilled and channel does not support partial cancellation" while trying to mark an Order COMPLETED on Search Orders screen.

Most of the channels won't allow partial cancellation (For B2C Orders). A B2C Order can only marked as Completed once all items got packed and invoiced. If none of the picked items are Packed and Invoiced, user could mark the Order as Cancelled instead of marking it as Completed.

"Error in closing shipment: Error creating order from rms, error message: Internal error"

(Check the logs and raise Jira ) This error occurs if there is mismatch in the ZIP code. So confirm the correct zipcode and correct in OMS for the orders which are already synced  write the query and update it.