One of the new week 3 challenges in Fortnite requires you to watch a replay match from your library. As of now the replay challenge can’t be completed because epic had to delay the new 4.2 update. Checkt they’re twitter for updates.
Once the new update is live you can simply go to carrer-replays and complete the challenge by watching one of your latest replays.
The Replay challenge was removed with update 4.2 and replaced with a challenge to review 5 teammates.