https://userapps.support.sap.com/sap/support/knowledge/en/3460235
3460235 - BAPI_PO_CHANGE: Error 06 070 - Enter a quantity - SAP ERP and SAP S/4HANA | SAP Knowledge...
In the BAPI_PO_CHANGE, you receive the error message 06 070 - "Enter a quantity", when you delete all the schedule lines of the purchase order.