C/Visual C - No se cual es el error en SUBTITUTION CS50 el link esta en la description

#include <cs50.h>

#include <stdio.h>

#include <string.h>

#include <ctype.h>

int main(int argc, string argv[])

{

    // check that the lenght of the argc is 2, if not print ./substitucion KEY

    if (argc != 2)

    {

        printf("./substitucion KEY\n");

        return 1;

    }

    // Check that the lenght of the key is == 26 if not, print THE KEY MUST BE 26

    if (strlen(argv[1]) != 26)

    {

        printf("The key must contain 26 letters\n");

        return 1;

    }

    // SAVE THE KEY AS KEY ARV[1] = KEY

    string key = argv[1];

    // check if the key is alphabetical

    for (int x = 0; x < strlen(key); x++)

    {

        if (!isalpha(key[x]))

        {

            printf("It must contain only alphabetical letters\n");

            return 1;

        }

    }

    // Check that each letter of the key is unique

    for (int y = 0; y < strlen(key); y ++)

    {

        for (int u = y + 1; u < strlen(key); u++)

        {

            if (toupper(key[y]) == toupper(key))

            {

                printf("The key alphabet can only be repeated once\n");

                return 1;

            }

        }

    }

    // ask the user ewhat will be the plain text

    string plaintext = get_string("plaintext: ");

    printf("ciphertext: ");

    // encypher the plain text using an algorithm and print the cyphertext

    for (int i = 0, len = strlen(plaintext) ; i <= len; i ++)

    {

        if (isupper(plaintext[i]))

        {

            int INDEX = (plaintext[i] - 'A');

            printf("%c", toupper(key[INDEX]));

        }

        else if (islower(plaintext[i]))

        {

            int index = (plaintext[i] - 'a');

            printf("%c", tolower(key[index]));

        }

        else

        {

            printf("%c", plaintext[i]);

        }

    }

printf("\n");

}