DLL Properties grayed out (unselectable), unable to exclude plugin from platforms

I am getting a build error when using the Unity Robotics URDF Importer from a duplicate DLL in the package. Unity Robotics has advised I select DLL in the editor and edit it's properties to exclude it from the Android Platform.

HOWEVER, When I select that particular DLL, all of the properties are grayed out and I am unable to deselect the 'Any Platform' checkbox. (Screenshot attached below). I noticed this DLL is 'Managed', is there somewhere else I Am supposed to be editing the platform exclusions from?