Can I enter a Support a Creator code if I subscribe to Fortnite Crew?

No, Support-A-Creator codes cannot be used to purchase the Fortnite Crew. They can be used in the Fortnite Item Shop and apply to in-game purchases made with either earned V-Bucks or purchased V-Bucks, as well as most real-money offers.

To declare your support for your favorite creator, please see this article: How to add a Creator Code in the Fortnite Item Shop?