Hi
When we assign a FD to sale order and it is not accepted by system due to any reason and system gives warning message Message no. EI574, then it means that order is blocked and some user had released it through VKM3. In your case aren't you getting this order block? Why would you need to make this message as error when in standard system is blocking the order and you can't create delivery and can't do PGI unless some authorize user releases this order.
Thank$