https://www.gamespot.com/articles/fortnite-birthday-challenges-not-showing-up-for-so/1100-6460671/

Update: Although the new Fortnite patch is out now and the birthday event is live, the new challenges are not available for everyone as intended. Epic has confirmed issues have arisen with the rollout, causing "some" players to be unable to access the challenges. However, that should be fixed before long; Epic says everyone will get the challenges when the in-game store refreshes today, July 24, at 5 PM PT / 8 PM ET (1 AM BST on July 25). The original story follows.

A new update is on the way soon for Fortnite: Battle Royale. Coinciding with the game's first anniversary, Epic is rolling out the v5.10 patch on all platforms--PS4, Xbox One, PC, Nintendo Switch, and iOS--today, July 24, which will kick off a special, limited-time birthday event and introduce other new content to the battle royale game. [Update: The servers are now back online. You can read the full Fortnite 5.10 patch notes and our Fortnite 1st Birthday challenge guide to see everything that's new and what you'll need to do to get some fresh cosmetics.]