Stripe Cashier | Call to a member function swap() on null

I'm trying to swap() price with Cashier, but I get Call to a member function swap() on null . Here is what I did with Laravel 11:

• I bought a new plan for this user | the plan is reflected in Stripe Dashboard and in both tables: subscriptions and subscription_items.
• I swap() product
• I get the error

Thanks

Log:

local.ERROR: Call to a member function swap() on null 
 

MainController

$req= $request->user() ->subscription('default')->swap($price);