CATS Blender Addon - Trouble Attaching Mesh to Armature

Hi! Blender/Unity newbie making my first avatar here.
I’m using the latest version of the CATS addon in Blender 2.80. I’m trying to attach a simple collar mesh to my avatar’s neck, but when I select the base, mesh, and bone to attach to, I get the following message, and the collar disappears:

I’m a beginner, so I’ve been relying on tutorials and such and poking around to learn what I need to, but I’m pretty stumped on this.

try Alt+P -> Clear Parent on mesh that you want to attach