>>2649261Dev here, didn't expect anyone to stumble across that message lol
I'm guessing you need to apply a Humanoid rig to the fbx first, by default fbx will have a generic rig. The plugin needs the Humanoid rig to do its bone renaming magic.
Click on the fbx in the asset browser (not scene hierarchy) then in the inspector go to the Rig tab.
Change Animation Type to Humanoid and click Apply.
Click on the Configure button that pops up and verify the bone mapping. (you might need to refresh the inspector by clicking on another asset first)
Usually Unity does a decent job with the automapping but sometimes you need to manually update the mapping.
After that the fbx should go through the exporter normally.
If it doesn't work, do you mind to share the fbx you are trying so I can take a look?