I have made a rig for a model, however the main torso piece does not attach to the rig due to this error. I have sought out various solutions but none seem to work. Does anyone know the issue? My Blender Project
The bone heat is failing because of three reasons: there are lots of doubles, the mesh is not a single piece and there is overlapping geometry.
Each of theese facts cannot work with the automatic weights feature.
And automatic weighting is not what you want in this kind of characters, because it would distribute the weight between different bones also in parts of the clothes that should be rigid and metallic, deforming them.
So, the way to go is definitely hand weighting: first of all give an appropriate name to the bones, then in edit mode select a spare part, select its appropriate bone/vertex group and click assign.