The type or namespace name "Core" could not be found: MVVMCross Error

I followed the guides for creating a MVVMCross project carefully but it seems that I can't make it work. I haven't touched any code, just followed the instructions then I've built the project. But the error persists in "return new Core.App()"

using Android.Content;
using MvvmCross.Droid.Platform;
using MvvmCross.Core.ViewModels;
using MvvmCross.Platform.Platform;

namespace HelloMVVM.Droid
{
    public class Setup : MvxAndroidSetup
    {
        public Setup(Context applicationContext) : base(applicationContext)
        {
        }

        protected override IMvxApplication CreateApp()
        {
            return new Core.App(); // The error appears here
        }

        protected override IMvxTrace CreateDebugTrace()
        {
            return new DebugTrace();
        }
    }
}