https://www.polygon.com/2018/6/12/17453978/this-fortnite-account-is-associated-with-a-platform-which-does-not-allow-it-to-operate-on-switch

Fortnite launched on the Nintendo Switch today, and that version of the game can play against folks on iOS, Mac, Windows PC and Xbox One. The problem is that using a Fortnite account that has been used on the PlayStation 4 will keep you from using that account on the Switch, and an account that has been used on the Switch can’t be played on the PS4.

If you’ve played on PS4, the Switch version will give you the following error message: “This Fortnite account is associated with a platform which does not allow it to operate on Switch.”