Solution : https://service.sap.com/sap/support/notes/350226 (SAP Service marketplace login required)
Key words : 
additional key words apo, apo backorder processing, prerequisites missing functions, solution prerequisites, availability check, confirmed quantities, notes 321702
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting | 
| 321913 | Transfer of the backorder processing results | 
| 321909 | Structure - results of backorder processing | 
| 321702 | Transfer of results of APO backorder processing |