Fortnite is free to play, right? Not necessarily on all platforms.
It's officially free to play on PC from the Epic Games store, but not on all platforms. Xbox and PlayStation 4 players need a membership to access any online game, including Fortnite.
Xbox locks down its Xbox Live Gold online multiplayer content. Xbox Live is split into two tiers: Free and Gold. If you want to play online, you will have to pay for this Gold Membership. There are a few other upgrades available, including Game Pass and Ultimate, but if you just want to play online with friends, Xbox Live Gold is the cheapest option.
Sony does the same with its PlayStation 4 console. PlayStation Plus is Sony's version of Xbox Live without the extra levels. The price of the program is about the same and with the option to be billed monthly, quarterly and annually. If you want to play online, you have to pay at least a monthly subscription to do so.
Nintendo Switch is the only console that does not charge its players a fee to play with their friends in Fortnite. Nintendo has a subscription service, Nintendo Switch Online, but Fortnite remains free to play on the Switch.
So do you need an Xbox Live membership for Fortnite? Yes. And you need a PlayStation Plus subscription on PS4. If you want to avoid subscription fees, you have the choice between the Nintendo Switch or the PC.