saying error on line 6 but this is pre determined code. What am I missing?

using System;

class Program
{

    static string CheckSpeed(double speed)
    {

        if (speed > 65)
        {
         Console.WriteLine("too fast");
        }

        else if (speed < 45)
        {
         Console.WriteLine("too slow");   
        }

        else
        {
         Console.WriteLine("speed ok");   
        }
    }

    static void Main(string[] args)
    {
        // This should print "too slow".
        Console.WriteLine(CheckSpeed(44));
        // This should print "too fast".
        Console.WriteLine(CheckSpeed(88));
        // This should print "speed OK".
        Console.WriteLine(CheckSpeed(55));
    }

}