The problem here is that you’re sealing your mesh with faces have more than 4 vertices, also known as n-gons. While they render fine in Blender, when exported into a game engine like Roblox the engine will try to triangulate all of the faces on the mesh. This is where it runs into issues like your first example, because it gets confused on how to turn those n-gons into a bunch of triangles!
You should be redoing the entire topology of those n-gons by hand and properly linking those faces together, but that is a time-consuming task and might even require you to learn a new skill. For the time being, you could try selecting your n-gons and hitting “Triangulate Faces” under one of the tabs in Edit Mode and seeing if that will fix your problem!
The invisible parts are gone! (Just on the little teeth things)
The thing is that I don’t want to triangulate everything because I’m planning on using real textures instead of painting on it or just using Roblox’s texture. And I’m still terrible at UV Texturing so it becomes very hard when there are too many faces, this means I’ll have to triangulate just that part manually
BUT IT WORKS!!!
Thank you so much.