https://www.gamespot.com/articles/fortnite-challenges-where-are-the-rift-portal-loca/1100-6461051/

Season 5, Week 5 of Fortnite: Battle Royale is here, and with it comes a few new challenges to complete. One of those will require you to run around the game world in search of Rift Portals to use, which is much easier said than done. Rifts played a key part in the lead up to Season 5, transforming large areas of Fortnite's island. Interacting with them teleports you high into the air above the island, allowing you to plummet down to the ground or use the elevation to glide around. [Update: A bug that caused Epic to remove Rifts from the game temporarily has been resolved, so you should be able to once again undertake the challenge explained below.]

The difficult part of this challenge is actually finding these portals, as they appear randomly. Generally, you'll have to run around and just keep your eyes open, so we recommend just getting on with the other challenges and staying alert. However, there is one extra tell to look out for: when a rift opens, you'll hear a booming sound that serves as an audio cue to alert players nearby that one has opened, so keep your headphones on and turned up.