predefined type System.Range is not defined or imported

Question

I am using C# 8.0 (beta) in my UWP app with Visual Studio 2019 v16.0.2. I was trying to use the new range feature of C# 8 like str[start..index] and then two error showed up:

1. Predefined type 'System.Range' is not defined or imported
2. Predefined type 'System.Index' is not defined or imported

But there is no such reference that I could use. Visual Studio itself suggested me to use this feature. Is that the feature hasn't made available yet or what?