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

With the start of Fortnite Season 5, Week 5 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 much easier said than done. Rifts were a key part of the start of Season 5, as they transformed large areas of the 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.

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, there is 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.