CreateITShopOrder method throwing Error for Multirequestable and Unsubscribable resource (QERReuseUS)

Hi Team,
I have manually assigned the users to the business roles(PersonInOrg) which is Multirequestable and Unsubscribable. Just to have reference of users access i wanted to create pwo entry. When used the CreateITShopOrder method for pwo creation getting below error

Error Message:

Error during execution of 'OnSaving' in logic module 'QER.Customizer.AssignmentITShopOrderEntityLogic'
Assignment requests are not allowed for the given product nodes.

Code: 
Public Function CreatePWO(ByVal xobject as string) as string

Dim baseobject as IEntity = session.source.Get(New SingleDbObject(xobject))
.
.
.
baseobject.Custom.CallMethod("CreateITShopOrder", strUID_ITShopOrg, strUID_Person,createMultirequest_PWO)
.
End Function

Public Sub createMultirequest_PWO(ByRef dbAssignment As IEntity, ByRef dbPWO As IEntity) As String

dbPWO.PutValue("OrderReason", "Role request assignment converted ITShop request.")
dbPWO.PutValue("DateHead", Now().ToString)

End Sub

Please help to fix this issue