cs50 pset2 vignere error

#include <ctype.h>
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#define ALPHABET 26

int main(int argc, string argv[])
{
    if (argc !=2)                                                                       // invalid if there are more than two arguments
    {
        printf("Enter two arguments only.\n");
        return 1;
    }
    else
    {
        for (int i = 0, j = strlen(argv[1]); i < j; i++)                                // loop through each character of key to ensure its all alphabets
        {
            if (isalpha(argv[1][i]))
            {
                printf("Re-enter key with alphabets only.\n");
                return 1;
            }
        }
    }

    string key = argv[1];                                                               // bind argument 1 to key variable
    string plaintext = get_string("plaintext: \n");                                     // prompt user for plaintext

    for (int i = 0, int j = 0, int pLength = strlen(plaintext); i < pLength; i++)
    {

        int keyLength = strlen(key);                                                    // get length of key
        char pLetter = plaintext[i];                                                    // bind each character of plaintext to pLetter
        char kLetter = argv[1][j % keyLength];                                          // bind each character of key to kLetter

        if (isupper(kLetter))                                                           // get alphabet index of key if its uppercase
        {
            kLetter -= 65;
        }

        else if (islower(kLetter))                                                      // get alphabet index of key if its lowercase
        {
            kLetter -= 97;
        }

        if (isupper(plaintext[i]))                                                      // add key to upper plaintext character, deduct 65, module by 26 and add back 65 to get ASCII value
        {
            printf("%c", (((plaintext[i] + kLetter - 65) % ALPHABET) + 65);
            j++;                                                                        // increment j by 1 to loop through each character of key
        }

        else if (islower(plaintext[i]))                                                 // add key to lower plaintext character, deduct 97, module by 26 and add back 97 to get ASCII value
        {
            printf("%c", (((plaintext[i] + kLetter - 97) % ALPHABET) + 97);
            j++;
        }
        else
        {
            printf("%c", plaintext[i]);                                                 // if plaintext is neither upper or lower, print character as is
        }
    }
    printf("\n");
}