It sounds like you need to add a Quantity to the Output Quantity field.
If this Production Order was a mistake, as suggest previously, if nothing has been posted to the Production Order and it was a mistake, simply delete the Production Order. If postings have already happened, and it was a mistake, you generally want to back out the entries (same posting but with a negative quantity), then post an Output Quantity of 1 and Finish the Production Order. Don't forget to do a negative adjustment to inventory to clear out the 1 you output (again, assuming it was a mistake).
If this Production Order was not a mistake, you would want to post whatever quantity was actually output from the Production Order. Once the Output Quantity > 1 the system will allow you to Finish the Production Order.
The Finish process if final! I cannot stress this enough. Be 100% sure all consumption and output have been posted against the Production Order before you change status to Finished. You cannot undo the process.
