PO creation From accepted RFx response

Hi,

Please can you help ME, i am not able to create PO of services, here is my process :

SC created in SRM - > SC goes to sourcing -> buyer creates RFx -> RFx is sent to bidder -> bidder processes RFx Response -> buyer accepts RFx response -> buyer creates follow on document (PO) from RFx Response (create PO).

Error message:  /SAPPSSRM/HIER 015

Product type XXX  of item differs from type XXX of product XXX.

N.B : PO based on materials number work fine.

Thank you,

Karim