Solution : https://service.sap.com/sap/support/notes/480649 (SAP Service marketplace login required)
Key words : 
backorder processing terminates, runtime error getwa_not_assigned, /sapapo/bop_resultbackorder processing, apo solution implement, source code repairs, terms transaction
Related Notes : 
       
| 481921 | Backorder processing: Runtime error GETWA_NOT_ASSIGNED | 
| 459748 | BOP, backorder processing: performance results display | 
| 375193 | Backorder processing: Composite SAP note on posting |