https://userapps.support.sap.com/sap/support/knowledge/en/3382739
3382739 - EDI: Error message M8607 raised for IDoc without PO account assignment data - SAP ERP &...
An IDoc of INVOIC type for blanket PO, without account assignment, is posted to create an invoice. The error message is triggered: M8607 System error: Error in...
https://userapps.support.sap.com/sap/support/knowledge/en/2417053
2417053 - Error M8607 for Service Purchase Order - - SAP ERP & S/4HANA | SAP Knowledge Base Article
For a service purchase order you get Error M8607: 'Error in routine MRM_FRSEG_CHECK ' for different transactions.