Constant negative refractive index

Why can I not set a structure to have a constant, negative refractive index?

Unless specified, we assume materials are non-magnetic, eg, the Dielectric material type. Therefore, it is not allowed to assign a constant negative refractive index. That said, we do allow users to simulate the effects like structure with a negative index. One way to do that is to make use of the Magnetic Electric Lorentz to define both permittivity and permeability. I would suggest you to take a look at this example and see if is useful to your applications (https://kb.lumerical.com/en/index.html?metamaterials_bulk_metamaterials.html).