Ambiguous errors when trying to compile C++ code

When trying to compile the code below I get three errors.

• 'iterator_category': is not a member of any direct or indirect base class of 'std::iterator_traits<_InIt>'
• '_Iter_cat_t' : Failed to specialize alias template
• type 'unknown-type' unexpected

I'm quite new to C++ and have gone over the code many times changing snippets but nothing helps. Any help deciphering these error messages is much appreciated.

#include "../../std_lib_facilities.h"

class Puzzle {
public:
    vector<char> letters;
    Puzzle(int my_size);
    void generate(void);
    void enter_letters(void);
    void feedback(Puzzle puzzle);

private:
    int size = 4;
};

Puzzle::Puzzle(int my_size)
{
    size = my_size;
}

//Generate size unique letters for the letters array.
void Puzzle::generate(void)
{
    for (int i = 0; i < size; ++i) {
        char rand = randint(26) - 1 + 'a';
        while ((find(letters[0], letters[size], rand) != letters[size])) {
            rand = randint(26) - 1 + 'a';
        }
        letters[i] = rand;
    }
}

//Let the user enter size unique letters.
void Puzzle::enter_letters(void)
{
    cout << "Enter four different letters seperated by spaces:\n";
    for (int i = 0; i < size; ++i) {
        char letter;
        cin >> letter;
        letters[i] = letter;
    }   
}

//Tell the user how many bulls and cows they got.
void Puzzle::feedback(Puzzle puzzle)
{
    int cows = 0, bulls = 0;

    for (int i = 0; i < size; i++) { //input
        for (int j = 0; j < size; ++j) { //puzzle
            if (i == j && letters[i] == puzzle.letters[j]) {
                ++bulls;
                break;
            }
            else if (letters[i] == puzzle.letters[j]) {
                ++cows;
                break;
            }
        }
    }
    cout << "Bulls: " << bulls << "\nCows: " << cows << "\n";
}

//Seed the random function.
void seed(void)
{
    int sum = 0;
    cout << "Enter a random string of characters:\n";
    string str;
    cin >> str;
    for (char& c : str) 
        sum += c;
    srand(sum);
}

int main()
{
    constexpr int GAME_SIZE = 4;
    seed();

    Puzzle puzzle(GAME_SIZE);
    puzzle.generate();
    Puzzle input(GAME_SIZE);

    input.enter_letters();
    while (puzzle.letters != input.letters) {
        input.feedback(puzzle);
        input.enter_letters();
    }
    cout << "Congragulations, you did it!\n";

    keep_window_open();
    return 0;
}