Error: Predefined type 'System.Range' is not defined or imported

Earlier I was getting the error:

Feature 'range operator' is not available in C# 7.3. Please use language version 8.0 or greater.

When I upgraded the project below error still remains:

Predefined type 'System.Range' is not defined or imported

Basically trying to test how can we retrieve a fragment of an array or string:

using System;
using System.Windows.Forms;

namespace TestRange
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var story = "Testing C# 8";
            Console.WriteLine(story[^6..^0]); // (last 6 chars)
        }
    }
}

Articles that I referred to but didn't help:

https://docs.microsoft.com/en-us/dotnet/api/system.range?view=netcore-3.1 https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/ranges https://docs.microsoft.com/en-us/dotnet/csharp/tutorials/ranges-indexes

https://www.infoworld.com/article/3532284/how-to-use-indices-and-ranges-in-csharp-80.html

https://www.codejourney.net/2019/02/csharp-8-slicing-indexes-ranges/

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