https://userapps.support.sap.com/sap/support/knowledge/en/3681256
3681256 - Inbound Delivery "GR for Production" created from COR6, but missing PPO reference in EWM....
Inbound Delivery "GR for Production" is created from tcode COR6, but there is no PPO reference in Reference Document tab and it is unable to display the PDI...