Hello
On transaction CO01 you can choose the production and planning plant during the order creation. Maybe your user chose the wrong plant by mistake.
Another possibility is that you are using a special procurement type at quota arrangement level.
I'm not sure if it is possible to post the goods receipt, but I strongly suggest you to cancel all the work carried out on this order and create a new one, with the correct plant, in order to avoid any kind of problems during costing or reporting in the future.
BR
Caetano