Hi there! This can happen as a result of upgrading your project from using MARS 1.0.x to 1.1.x. This type used to be a ScriptableObject and is now a static class. The old MarsUserPreferences.asset file is probably still in your project, and Unity is trying to create the backing object for it and you see this error as a result.
Just delete the offending file (should be under Assets/MARS/UserSettings) and the error should go away.