Yes, I am asking if the plugin can retarget to a non UE5 skeleton. But it seems like the plugin only works with UE5 skeleton?? ( planning to refund it 
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I think I found a video about retargeting Mixamo character to a non-UE5 skeleton Retarget mixamo animation and character in unreal engine 5 final - YouTube . I will try that out, and also seek for more other solutions.
Thank you so much!! 