https://userapps.support.sap.com/sap/support/knowledge/en/2845160
2845160 - BAPI BAPI_SALESORDER_SIMULATE doesn't show error for all the items in return table | SAP...
You are using BAPI BAPI_SALESORDER_SIMULATE to simulate sales order creation with multiple items. The problem is that this BAPI doesn't show error messages...