User cannot perform set operations on this line.

"User cannot perform set operations on this line", this is very common throw by Oracle Application when you try to add a Sales Order line to existing Fulfillment/Ship/Arrival Set. There are many reasons for this Error, but in this post I will discuss one such Case.

Steps -1.
Create Sales Order and Add a Model Line as well as Standard line in the Order .

Steps -2.
Expand the Model and navigate to the Class Line.

Steps -3.
Try to add the Class Line in a New or Existing Fulfillment Set and Boom , you get the error message "User cannot perform set operations on this line".

Reason - Why on Earth you want to add the Class to a Fulfillment Set (Note -Model are always in an Implicit Fulfillment Set ) , but if you want to add the Model and Non Model line in a Fulfillment set then Add the Model line and Non Model line , but do not try to add a Non - Model line and Child of Model ( Class/Option) to the Fulfillment Set as that is not Supported behavior and that also has no business need.

Please Note that When we add a model line to a Set , Set will propagate to all the childs of the Model , but this is not the case if you add a CLASS beneath a MODEL in the Fulfillment set.

Create Sales Order with Model  

 Add a Non-Model Line to Order ( I mean Standard line)

 Try to Add  Item of type Class (of Model) to Fulfillment Set

Error Message