Thank you @pfaffman.
I’m not sure I made myself understood.
Discourse Subscriptions allows site owners to sell recurring and one-time purchase subscriptions that grant access to a group on a Discourse instance.
So I therefore understand that only people who have paid can see and reply. What I’m looking for is to allow the creation of a new discussion in a specific category only for those who have paid. But all forum users can read and reply for free.