✅ Not understanding Avalonia Error

public void CreateNewUser(Database db, CountryCodes cc)
{
    var vm = new NewUserViewModel(cc);

    Observable.Merge(
        vm.Create
            .Do(model =>
            {
                _db.SaveNewContact(model);
                ShowHomeScreen(db, cc);
            }), 
        vm.Back)
        .Take(1)
        .Subscribe(model =>
        {
            if (model == null)
            {
                ShowHomeScreen(db, cc);
            }
        });

    Content = vm;
}