Gengar Top for Nintendo switch game cartridge pokeball

For starters this is my first remix model.. definitely still a lot to learn so please go easy. Really enjoyed the Nintendo switch game cartridge pokeball so I wanted to make a Gengar variation and try remixing my first model. When loaded to slicer model shows non-manifold warning however I am still able to print and have not figured how fix. If anyone has advice pls msg me! Enjoy!