If you've ever played Fortnite on PS4, you'll be blocked from playing the version.
Playing Fortnite requires you to log into an Epic Games account. If you've already connected the account to a PSN ID, you will be greeted with the following message when booting the game on Switch :
"This Fortnite account is associated with a platform which does not allow it to operate on Switch. Neither the Fortnite website nor Epic Customer Service are able to change this. To play Fortnite on Switch, please create a new account."