Hello Eike,
first, don't worry abouth the warnings in the activation of hte BC set. Most BCS for warehouse requests (deliveries, production material reqzuest, posting change...) get this warning because one of the view clusters that is used everywhere in those documents does not need content on all levels. The warning is about that. Just ignore it.
When the message from ERP fails with that error message, the root cause is that the item type that the system finds an item type that does not contain the necessary quantity roles and status. In IMG activity EWM -> Interfaces -> ERP Integration -> Production -> Map SAP ERP Order Type to SAP EWM Item Type make sure that your message from ERP finds item type PCSO.
Regards,
Katrin