Mesh Error?

So I used Sims 4 Studio and created a new package based on the Pristine Pedestal. I then exported the texture and mesh. I edited the texture and imported it fine. But when I edited the mesh, I did something a little different. I deleted the original object, creating a new cube object at 0,0,0. I then adjusted the width, depth, and height. I then imported it into my package. It worked, but there are these weird dark artifacts that make it look semi-transparent and hollow inside. The package doesn't have a diffusion or specular layer, so I'm not sure what's wrong. I also tried deleting two custom lights from the mesh to no avail. How do I get rid of these so that the box looks solid?

Imgur Post

https://i.imgur.com/pBs0HXm.png