unique character password generation in this code

private static string MakePassword(int pl) 
       {
           char[] comb = new char[pl];
           string possibles =
           "ZYXWVUTSRQPONMLKJIHGFEDCAB";
           char[] passwords = new char[pl];
           Random rd = new Random();
         
           try
           {


               char ss = possibles[rd.Next(0, possibles.Length)];
              char sp= possibles[rd.Next(0, possibles.Length)];
              if (ss != sp)
              {
                  char a = sp;
              }
               passwords[1] = possibles[rd.Next(0, possibles.Length)];
               passwords[2] = possibles[rd.Next(0, possibles.Length)];
               passwords[3] = possibles[rd.Next(0, possibles.Length)];
               passwords[4] = possibles[rd.Next(0, possibles.Length)];
               passwords[5] = possibles[rd.Next(0, possibles.Length)];
               passwords[6] = possibles[rd.Next(0, possibles.Length)];
               passwords[7] = possibles[rd.Next(0, possibles.Length)];
               passwords[8] = possibles[rd.Next(0, possibles.Length)];
               passwords[9] = possibles[rd.Next(0, possibles.Length)];
               passwords[10] = possibles[rd.Next(0, possibles.Length)];
               passwords[11] = possibles[rd.Next(0, possibles.Length)];
               passwords[12] = possibles[rd.Next(0, possibles.Length)];
               passwords[13] = possibles[rd.Next(0, possibles.Length)];
               passwords[14] = possibles[rd.Next(0, possibles.Length)];
               passwords[15] = possibles[rd.Next(0, possibles.Length)];
               passwords[16] = possibles[rd.Next(0, possibles.Length)];
               passwords[17] = possibles[rd.Next(0, possibles.Length)];
               passwords[18] = possibles[rd.Next(0, possibles.Length)];
               passwords[19] = possibles[rd.Next(0, possibles.Length)];
               passwords[20] = possibles[rd.Next(0, possibles.Length)];
               passwords[21] = possibles[rd.Next(0, possibles.Length)];
               passwords[22] = possibles[rd.Next(0, possibles.Length)];
               passwords[23] = possibles[rd.Next(0, possibles.Length)];
           }

           catch (IndexOutOfRangeException e)
           {
               for (int j = 0; j < pl; j++)
               {
                   comb[j] = passwords[j];
               }
           }
           return new string(comb);
       }