The figure you have selected uses weight-mapped rigging, assuming this a joint you want to edit? And you open the Tool Settings pane and see the area at the bottom for showing the list of maps? is it empty or not? If it is empty, is the button labelled Unused maps just above it? If you click the unused maps button are there any entries in the pop-up menu? If there are, can you select one so that it appears in the list of active maps?