How to fix objects that show up black in "Lumen Scene" view mode? How to enable nanite for FBX?

I really appreciate this information!! As a beginner, I’m not familiar with a lot of this, but I’ll look up the terms and try to make some headway toward a solution. Thank you so much! Your last suggestion makes me think, maybe the extremely high number of plants could be messing up my scene? I was told I have an incredibly huge amount of grass, which was bogging down Unreal, even slowing down camera moves. Maybe reducing the plant count will help with Lumen’s processing issues. In any case, I really appreciate your time!