I am fairly new to Unity, and create avatars for the game VRChat. When trying to upload the avatar, I got the error “LowerArm is not first child of UpperArm or Hand is not first child of LowerArm: you may have problems with forearm rotations”. I tried going into the bone mapping and made sure none of the arm bones were twist bones, tried everything but nothing worked. I left a screenshot of my arm hierarchy. 
I don’t know how to help you but, if you haven’t tried it already, you may also find help in the VRChat forums: https://ask.vrchat.com/c/avatars/26
ok