Why I can't Fake-Mock "System" in .net core 6.0(VS Enterprise 2022)

Why I can't Fake-Mock "System" in .net core 6.0(VS Enterprise 2022), I don't know what happened?

I was trying to use "using System.Fakes; " , But it alert "missing an assemly reference" after following

https://learn.microsoft.com/en-us/visualstudio/test/isolating-code-under-test-with-microsoft-fakes?view=vs-2022&tabs=csharp 

but I can see "System.fakes" file in my "Fakes" folder?

Please provide a working example in .net core for my question.

My code as below:

using System.Fakes; 

[TestClass]
public class MockDateTimeTest{
[TestMethod]
public async void TestTodayIsLastDateOfYear(){
// Arrange

// Actbool result = false;
using (ShimsContext.Create()){

ShimDateTime.NowGet = () => new DateTime(2013, 12, 31);
result = DateTimeHelper.IsTodayLastDateOfYear();
}

// Assert
Assert.IsTrue(result);
} 

}

public static class DateTimeHelper{
public static bool IsTodayLastDateOfYear()
{DateTime today = DateTime.Now;
if (today.Month == 12 && today.Day == 31)
return true;elsereturn false;
}
}

Let me know if you can use "using System.Fakes" in .net core successfully and please answer me as soon as possible.

Thanks in advance.