Limit PO :Combination of GR/IR is not allowed for external services

Hi,

we are working in SRM40 sp09 & back end ECC.50

In the Limit PO:

we are able to create Limit PO from a shopping cart.The follow on documents in ECC, it is creating a PO with Item category "B" and the "invoices" are only possible.

The Issue:

The Client wants the Confirmation/GR functionality to be enabled and the Invoices.

but for Limit PO, with follow on documents--"Confirmation" is checked, we have a error message—“combination of GR/IR is not allowed for external services".

Is there any addition configuration or steps involved.

Thanks and Regards,

RK.