I'm still having this issue with activating Nanite in Fortnite for over a month. It began with Fortnite update in August. My RX 7600 was running well with Ray Tracing in Fortnite before. A week ago I've asked Epic Games support for help and as for today there's no solution. They've just proposed to launch Fortnite with disabled Nanite function. Of course I tried everything they recommended and everything I could find on Youtube and games forums about this issue fixing. No way!