I can't turn on 120 FPS in Fortnite on Xbox
If you are having issues with turning on 120 FPS on Xbox, please first make sure that your settings are correct by checking out this article: How do I enable 120 FPS in Fortnite on my PlayStation 5 or Xbox Series X|S?
If you still can't enable 120 FPS, please submit an in-game bug report so that we can look into this issue. You can follow these instructions: How to submit a Fortnite bug report in-game