Animation for Rocketbox characters

Hi,

following this very simple tutorial https://www.youtube.com/watch?v=gqJTjuyNfh8

I try to animate some characters from rocketbox repository https://github.com/microsoft/Microsoft-Rocketbox

The problem is that when I try to drag and drop the fbx, Mixamo raise an error dialog that tells that is not able to automatic rig. From the tutorial instead it looks ok, showing the next panel as the operation is successful. 

Any suggestion?