Solution : https://service.sap.com/sap/support/notes/377640 (SAP Service marketplace login required)
Summary : 
      When creating an assembly order and adding a directly manufactured component to a production order, error CO684 occurs, indicating an inability to assign to make-to-order stock. This is due to a program bug. For resolution, firstly ensure Note 366404 is applied. Implement Note 366535 for versions 4.5 and 4.6. Modify the function module CO_KO_ORDER_ENTRY_EXT by adding the import parameter FLG_COLORD_EXPAND with specific type settings. Perform the prescribed source code corrections followed by the application of Note 387733 to complete the fix.
Key words : 
system generates termination co684, add import parameter flg_colord_expand, reference type rc27s-flg_colord_expand, directly manufactured component, settlement rule reason, function module co_ko_order_entry_ext, type specification, assembly order, production order, -order stock'
Related Notes : 
       
| 625170 | Condition EK02 incorrect for assembly order network | 
| 429273 | Program termination when extending subnetwork | 
| 387733 | CO630 when creating assembly order with network | 
| 366535 | Missing homogeneity list when extending subnetwork | 
| 366404 | Termination when enhancing subnetwork |