in standard movement 551 is very much supported it look like your system is configured ( IDoc WMTORD )only to receive the information into 999. After this ideally you are supposed to use transaction MIGO goods issue with movement type 551 even you can automate this part by just configuring / executing another IDoc MBGMCR with movement type 551
please check your business blueprint document / training document why it was suggested to use LI21