C# 6.0 - Unexpected character '$'

I'm new to programming and I have just installed Visual Studio 2017. I created this code (from the book I'm learning), but this does not compile. I have problem with string interpolation and I get error:

Unexpected character '$',

but I'm using C# 6.0 so this should not be a problem ?

static void Main(string[] args)   
{   
     string comparison;   
     WriteLine("Enter the number:");   
     double var1 = ToDouble(ReadLine());   
     WriteLine("Enter another number :");   
     double var2 = ToDouble(ReadLine());   
     if (var1 < var2)   
         comparison = "less than";   
     else   
     {   
         if (var1 == var2)   
             comparison = "equal to";   
         else   
             comparison = "greater than";    
      }   

     WriteLine($ "The first number is {comparison} the second number");
     ReadKey();   
}