Armature braking

When im make something (make less polygons with CATS and press FIX MODEL button), all my bones when im pasting even a regular 3d cube inside the bone, ALL bones weld into cube -
–i turn into a rock on the ground with model texture
–all bones move into one position
–Model breaking to parts

Is this after opening the FBX in blender, then re-exporting it back to Unity?

If so, it’s likely that there is different scaling on the original FBX and the one that you exported.

Try dragging the newly exported FBX into Unity and see if it looks fine - if so, you can just set up the avatar using that new model.

The alternative is fiddling with the export settings in blender to make sure that the scaling doesn’t change when you export. I think that CATS will make some changes to your export settings if you click the “Export” button in the CATS tool.