Error: Debug Assertion Failed! (string subscript out of range) (Hangman Game)

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I am having this huge issue with my Hangman Game program of a string being "out of range"...

What I can interpret about this is that something of the string has apparently an infinite number, furthermore, it can't store the letter you inputted to guess.

This is my Hangman code: (for further details)

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#include <iostream>
#include <string>    

using namespace std;
 string Palabra;

 int main() { 

cout << "Bienvenido a 'Hangman'\n\nPor favor, entre su palabra para 
adivinar: ";

getline(cin, Palabra);

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Some nice introduction;

Here I am trying to get (user input of the "Word or phrase") to move on the next section.

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int misses = 0;
int exposed = 0;

string display = Palabra;

for (int i = 0; i < display.length(); i++)
 display[i] = '_';

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"Misses" is the count of fails

"Loop For" Depending how long the Word will be, those will be replaced by Underline char. (Palabra is the same as Display)

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After this, some are self-explanatory... but still I will explain.

When you guess correctly a letter, the underline (display) will be filled up with a letter of the user inputted Word (Palabra)

If you repeat, it will tell you that already that letter exists in the Word (Palabra)

If you fail to guess a letter, it will tell you "That letter doesn't exist" in the word

If you completely lost all six tries, it will show you the Word that was supposed to guess.

   while (exposed < Palabra.length()) {
    { cout << "Miss: " << misses << ": ";
    cout << "Adivina las " << display << " letras de la palabra o frase.\nTienes " << misses << " intentos.";
    char response;
    cin >> response;

    bool goodGuess = false;
    bool duplicate = false;
    for (int i = 0; i, Palabra.length(); i++)
        if (response == Palabra[i])
            if (display[i] == Palabra[i])
            {
                cout << response << " esta en la palabra ya.\n";
                duplicate = true;
                break;
            }
            else {
                display[i] = Palabra[i];
                exposed++;
                goodGuess = true;
            }

            if (duplicate)
                continue;
            if (!goodGuess) {
                misses++;
                {
                    cout << response << " no esta en la palabra.\n";
                }
            }
            cout << "Si, la palabra fue " << Palabra << "." << endl;
            return misses;
    }
}
    system("pause");
}

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I can confirm that it works after the [getline (cin,Palabra)], but when I put a letter (for the guessing) , the error of "string subcript out of range" appears!

What's happening here?