Halion is a sampler and synthesizer that has been around since the early 2000s. Many sample libraries have been made for it, and only Halion can load these old libraries, including the marimba one you bought.
Halion Sonic and Halion Sonic SE are romplers and synthesizers, and until recently, they could only load libraries made by Steinberg.
Recently, Steinberg added the ability for Halion users to build libraries that work with Halion Sonic and Halion Sonic SE. They can’t load old Halion libraries, you have to build a library selecting the option to make it Halion Sonic (or SE) or higher compatible.
If only Halion is mentioned, assume that it wont work with any version of Halion Sonic.
I missed the fact that you’re using a Mac. Cubase 9.5 greatly improves the graphics performance on MacOS. I recommend upgrading at some point.