Solution : https://service.sap.com/sap/support/notes/110624 (SAP Service marketplace login required)
Key words : 
additional key words bapi_requistion_create, external number assignment, business object bus2105, advance correction refer, purchase requisition number, purchase requisition, bapi interface, method 'purchaserequisition, program error, bapi_requisition_create
Related Notes : 
       
| 503196 | Enterprise Buyer 3.5 Installation: Composite Note | 
| 409263 | Enterprise Buyer 3.0 Installation: Composite Note | 
| 360142 | Enterprise Buyer 2.0 Installation: Composite Note | 
| 303515 | SAP Bus.-to-Bus. 2.0B Installation: Composite Note | 
| 185164 | SAP Bus.-to-Bus. 2.0A Installation: Composite Note |