Solution : https://service.sap.com/sap/support/notes/362158 (SAP Service marketplace login required)
Key words : 
additional key words co11, error m7021 'deficit, goods movement overview, program error occurs, function module 'mb_create_goods_movement', solution program correction, stock fallen short, automatic stock determination, stock determination, required stock
Related Notes : 
       
| 392601 | Confirmation: M7021, incorrect quantity specification |