Hi,
Please clarify, for your entire production process no routing is required for all SFG and FG,You have assigned a default operation and then assign workcenter to it in Production order.
If yes then Go to T code OKKN and in quantity structure > remove the routing selection ID or else go to OPR4_CK , change message CK229 to warning from error.
Regards,
Anupam Sharma