GR allowed and expected for service PO &1/&2 but not "GR-based IV"

What causes this issue?

For the US federal government solution, if a service purchase order (PO)
has the goods receipt (GR) indicator (WEPOS indicator) meaning that GR
are expected and allowed from the order item but it is not GR-based IV
(WEBRE indicator); then an invoice for the order item can not be entered
with reference to a goods receipt document.
For service PO#s, it is crucial to have GR-based IV order items because
the sequential account assignment from the PO can be different to the
Service Entry Sheet (SES) and by having GR-based IV, MM passed the SES
number as reference that is stored in FM and the commitment string to
calculate the payment figures.
Service PO that have the GR indicator but are not GR-based IV do not
have the SES as reference so the payment calculation has to relay on
comparison of the coding block objects available in both FM and Split
Processor, as a result, the payment calculation cannot be guaranteed to
be reliable.
This is a customizable error message, SAP advices not to allow service
PO order items with GR that are not GR-based IV; default value is error
but it still can be changed to warning.

System Response

Service purchase order that have GR indicator should be "GR-based IV".

How to fix this error?

Modify purchase order or change the customizable error message
FMFG_BLEXT019 from type 'E' to 'W'.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.