Is something at your end. Body group tool works fine here. For what cause it. Hard to say when I can't re create the error here. Can be another addon causing a conflict, but not very likely. Not that familiar with the console error messages. Google was of little help to.
Try uninstall body group tool and re subscribe to it. Do the same with bone merger. Just a theory I have.
Remember to restart Gmod after uninstalling before re installing them.