why cant load "bitverse-unity-gui"?

This error appears to me while im trying to load the "bitverse-unity-gui".

Please tell what to do.

"Assets/package/bit-gui/system/controls/text/BitPasswordField.cs(36,32): error CS0122: `UnityEngine.GUI.PasswordFieldGetStrToShow(string, char)' is inaccessible due to its protection level"

It is said that "Private APIs can no longer be accessed in Unity 3" I failed to load that too. What a pity.