I have gotten past this problem. I'm afraid the answer isn't very enlightening. I think my configuration of the packages in the Unity Package Manager was just not correct. At least it seemed to just start working without doing any significant steps. I'm working with a plug-in Unity project that uses the Apple Unity Plugins. This plugin project is then referenced, by a Unity game project. I believe the issue was that the game project wasn't properly referencing the built tar packages. So now both the plugin project and game project are referencing the Apple Unity Plugin packages through pointing to their tarballs.