If those faces are planes, which means, that dont have thickness, one of the 2 sides will be facing the wrong direction, the solution is
- Solidify modifier
- Extrude that face, and then recalculate normals again
Also, roblox sometimes bug and even though in Blender faces are good, in roblox can be facing the wrong way, and it can be really annoying sometimes, so make sure that atleast, its not your fault, and in case blender its fine but roblox isn’t, just find another way to fix it.
And also consider making meshes with good topology, your mesh definitely is a “Mess” I know it is meant to be like this, but it is hard for Blender to do things correctly with this complex shapes, always try to keep square faces (4 vertices) and also in case nothing fix it, separate the mesh in more “simple” shapes, to the faces thing, and then, join them together again.
Topology is a hard term and usually it takes time and experience to model in a clean and good way for modifiers.