In order to use the Swap action for your purchased products, you must create a Constraint Rule of Action Type as Replacement. This rule fetches the available product in exchange of the product that you want to swap. Specify the product which you want to swap for in the Constraint Rule Condition and the product with which you want to swap to in the Constraint Rule Action.

CPQ enables replacement rules only on the ABO carts, for swap and split-swap functionalities.

For more information, see Managing Constraint Rules