This is a great tip from @EducraftIdeas that we may often be overlooked and find ourselves in the middle of trouble when uploading our model in FBX format to Mixamo. The problem that often occurred is the Error message “Sorry, unable to map your existing skeleton”. This happen to me a lot, so I dug up and looked around until a solution was found.

Thank you @EducraftIdeas, watch the video below and say thanks to the author.