It's possible to dance in Fortnite, and it is sometimes part of the weekly challenge. In order to dance, you must first equip a dance emote in your Locker from the lobby screen. All players have a standard dance emote, but it's also possible to get "better" dance moves by playing the game. Anyway, equip a dance emote, and head into a game.
The exact method to dance is a bit different depending on your console, so we have broken it down into the different ones:
Xbox One and PS4: Press down on the D-pad and select your dance emote with the right analog stick. Now just press A on the XB1 or X on the PS4 controller to begin dancing.
PC: Press B on your keyboard to open the emote menu, select the dance emote with your mouse, and click on it to begin the dance.
Nintendo Switch: Press down on the D-Pad, and select the dance emote with your right analog stick. Hit A to begin the dance.
And that's it! By following this guide you will be able to dance on any console, and you should be able to get your dance challenges done easily.