error: passing 'const Randomize' as 'this' argument discards qualifiers [-fpermissive]
              x = randomize1(index, size);
                                        ^
  int randomize1 (int x, int size)
  {
    int z = qrand() % size;
    if (x==z) randomize1(x, size);
      return z;
  }
  QString GetRandomString() const
  {
     const QString
     possibleCharacters("abcdefghijklmnopqrstuvwxyz");
     const int randomStringLength = 12; // assuming you want random strings of 12 characters

     int x = -1;
 int size, index;
 QString randomString;
 for(int i=0; i<randomStringLength; ++i)
 {
     size = possibleCharacters.size();
     index = qrand() % size;
     if (x==index)
       {
         x = randomize1(index, size); // ОШИБКА ЗДЕСЬ
       }

         QChar nextChar = possibleCharacters.at(x);
         randomString.append(nextChar);

         qsrand(QTime::currentTime().msec());
         Sleep::msleep(10);
     }
     return randomString;
  }