Solution : https://service.sap.com/sap/support/notes/1449703 (SAP Service marketplace login required)
Key words : 
run purchase order mass maintainence transaction - memasspo, memasspo call po bapi', po mass maintainence transaction, problem-specific key words, mass maintainence transaction, terms general keywords, -cwm specific note, solution apply note 1449476, prerequisites oil fields, present oil fields
Related Notes : 
       
| 1449476 | SAP_APPL : Creation of Enhancement points | 
| 443777 | |
| 417067 | |
| 414020 | MASS: Customer fields are not mass-maintainable | 
| 397391 | |
| 217231 | MEMASSPO: unchangble tab fields cannot be selected |