Simplify3D does tend not to be able to slice files that have multiple meshes that have errors in them......which is the case of the quoted file.
If you use either 3D Builder (Win10 app) or one of the online repair services to sort the file out, then you'll be able to slice the file in S3D.
You'll find that about 25% of the files you can freely download from the internet will need repairing before S3D will be able to slice them!