error when using ternary operator in formatting string (aka interpolated string) [duplicate]

I've written a simple program for finding the smallest permutation of a string which is lexicographically larger than the current one. But, the compiler emits the error

ERROR CS1003 Syntax error, ':' expected* ".

I use VS 2015 (update 3) and whenever I compile this program (which seems to be grammatically true), I encounter the aforementioned error.

Does this program have any error in syntax?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        const string a = "ABCDEFG";
        //var u = FFG(a);
        //var t = int.Parse(Console.ReadLine());
        //for (int i = 0; i < t; i++)
        //{
        //    Console.WriteLine(FFG(Console.ReadLine()));
        //}
        string u2 = a;
        string u = a;
        do
        {
            //***The follownig line meets Error***
            Console.WriteLine(u + $"{String.Compare(u, u2) > 0 ? true:false}");

        } while ((u = FFG(u)) != "no answer");
        Console.ReadLine();
    }
    static string FFG(string ss)
    {
        var s = ss.ToCharArray();
        int i = s.Length - 1;
        while (i >= 1 && s[i] <= s[(i--) - 1])
        { }
        if (i == 0 && s[0] >= s[1])
            return "no answer";
        int j = s.Length - 1;
        while (s[i] >= s[(j--)])
        { }
        j++;
        swap(s, i, j);
        int t = i + 1, tt = s.Length - 1;
        if (j - i >= 2)
            while (t < tt)
            {
                //if (t == j)
                //    t++;
                //if (tt == j)
                //    tt--;
                swap(s, t, tt);
                t++; tt--;
            }
        return new string(s);
    }
    static void swap<T>(T[] array, int i, int j)
    {
        T k = array[i];
        array[i] = array[j]; array[j] = k;
    }
}