Solution : https://service.sap.com/sap/support/notes/33078 (SAP Service marketplace login required)
Key words : 
additional key words vkm1, prerequisites program error, fixed order items, credit check, sales documents, credit representative, credit status, credit situation, credit limit, confirmed quantity
Related Notes : 
       
| 53201 | Availability check during release | 
| 35047 | Conf.qnty, credit limit check, transf.of reqs. | 
| 33103 | Credit check, availability check, performance | 
| 31248 | Release, another check and block, performance |