So I looked up ugcArtifactNotFound: Builder could not generate pcugc artifact. Check the material combinations are valid. Ensure texture file format is valid.
These are all 256x256 grayscale maps (even the base color map), so I'd done them in grayscale mode. Thinking that was the problem, I resaved them all in RGB mode. No joy.
These are the filenames, from what the guide says this is a valid combo: