I don't see the error when I try to add the legs and arms back to the SpineFixationWithFDK model. I am using AMS 8.0.1 with AMMR 3.0.1 and I suggest you try the same.
However, when you add the legs and arms back in the model, you will also need to provide the drivers for the legs and arms. You can do this by adding the default mannequin drivers in the study. Then, you would also need to use the overdeterminate solver as you also have some trunk drivers that will conflict with the mannequin drivers. Here are the lines you should add inside the study: