https://userapps.support.sap.com/sap/support/knowledge/en/3004297
3004297 - RFFOEDI1: Value of E1IDKU3-EXTPAYM is blank or incorrect | SAP Knowledge Base Article
Using program RFFOEDI1, payment medium Idocs with message type PAYEXT is created. In segment E1IDKU3 the value for field EXTPAYM is missing.