Nuitrack Automap Fail in Unity

I’m a beginner to Unity and Nuitrack, and I want to do body tracking on a human character model object, but when I click Automap, it shows the error below :

It is not possible to automatically fill in the skeleton map. Check the correctness of your model.
UnityEngine.Debug:LogError (object)
NuitrackSDKEditor.Avatar.NuitrackAvatarEditor:AutoMapping () (at Assets/NuitrackSDK/Avatar/3D Avatar/Scripts/Editor/NuitrackAvatarEditor.cs:358)
NuitrackSDKEditor.Avatar.NuitrackAvatarEditor:DrawAutomapTools (System.Collections.Generic.IEnumerable`1<nuitrack.JointType>) (at Assets/NuitrackSDK/Avatar/3D Avatar/Scripts/Editor/NuitrackAvatarEditor.cs:324)
NuitrackSDKEditor.Avatar.NuitrackAvatarEditor:DrawSkeletonMap () (at Assets/NuitrackSDK/Avatar/3D Avatar/Scripts/Editor/NuitrackAvatarEditor.cs:304)
NuitrackSDKEditor.Avatar.NuitrackAvatarEditor:DrawAvatarGUI () (at Assets/NuitrackSDK/Avatar/3D Avatar/Scripts/Editor/NuitrackAvatarEditor.cs:260)
NuitrackSDKEditor.Avatar.BaseAvatarEditor:DrawDefaultInspector () (at Assets/NuitrackSDK/Avatar/Scripts/Editor/BaseAvatarEditor.cs:18)
NuitrackSDKEditor.TrackedUserEditor:OnInspectorGUI () (at Assets/NuitrackSDK/Nuitrack/Scripts/UserManager/Editor/TrackedUserEditor.cs:14)
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

What could be the problem? Please help!