You are using bapi_po_create1 or bapi_po_change to add a new service line to one purchase order item with know account assignment category. Other terms
For the account assignment category you have set the G/L account as a mandatory field.
In the poaccount you are not filling the G/L account as you expect the system to get the G/L account automaticaly determined. After runing the bapi you will get no error message and the purchase order will be created without G/L account filled
ME21N, ML81N, ME51N, commitments Reason and Prerequisites
Program error Solution
https://service.sap.com/sap/support/notes/822621 (SAP Service marketplace login required)