Vigenere fails check50 test encrypts "BaRFoo" as "CaQGon" using "BaZ" error, my program output is "caQGoh".
I don't know what's the problem but i guess that this problem apears when there are different letter cases(lowercase and uppercase)plus the variance is about 6 shifts.
P.s : I am kind of new to programing and I am trying my best to use function.

Thanks for help :D

//this is my code
#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>


string getChars(string plaintext, string keyword)
{
int txtlen = strlen(plaintext);
int letter;
int j = 0;
for(int i = 0; i < txtlen; i++) 
{
    letter = plaintext[i];

   // check if it's a letter 
    if (isalpha(letter)) 
    {
    // encrypt if letter
        encryptChar(letter, keyword, j);
        j++;
    } 
    // if not just print it
    else 
    {
        printf("%c", letter);
    } 
}

printf("\n");
return 0;
}

char encryptChar(int letter, string keyword, int j)
{
int indexStart;
if (isupper(letter)) 
{
    indexStart = 65;
}

else 
{
    indexStart = 97;
}

char encrypted;
int keyLen = strlen(keyword);
//I guess down here is my problem.
int LtrNum = 0;
if (isupper(letter))
{
    LtrNum = keyword[j % keyLen] - 'A';
}
else if (islower(letter))
{
    LtrNum = keyword[j % keyLen] - 'a';
}
LtrNum = (((letter - indexStart) + LtrNum) % 26);
encrypted = LtrNum + indexStart;

printf("%c", encrypted);

return 0;
}
int main(int argc, string argv[])
{
string keyword = argv[1];
string plaintext = GetString();

// pass the text and the keyword to encrypt  
getChars(plaintext, keyword);

return 0;
}`