pset2 the last stretch

Question

    #include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, string argv[])
{
    if(argc != 2) // RETURNS TO 1 if not 2 strings
    {
    printf("Usage: ./caesar k\n");
    return 1;
    }  
    // calculate length of string and make sure it is alpha
    string keyphrase = argv[1];
    // k is the keyword wraparound integer get before loop so you can use k
    int k;
    // Get string length and calculate characters and loop string first for loop here
    for (int j = 0, l = strlen(keyphrase); j < l; j++)
        {
            if(!isalpha(keyphrase[j]))
            {
            printf("Please ensure all characters are alphabetical.\n");
            return 1;
            }
            else
            {
                k = j % l;
            }
        }

        // after string of key is validated get the printtext
                printf("plaintext: ");
                string s = get_string();
                printf("ciphertext: ");
    // CUT OFF HERE JUST IN CASE and also cipher plain text from here on out also second for loop starts here plain text is also variable s
    for(int i = 0, n = strlen(s); i < n; i++)
        {
            // NOT SURE WHAT TO DO TO MAKE KEYWORD WORK AND LOOP over each plaintext letter
            if(isupper(s[i]) && isupper(k)) 
            {
            // convert asci to alphbetical for upper case
            int cipher = s[i]-'A';
            int c = (cipher + k) % 26;
            printf("%c", c +'A');
            }

            //convert for lower case
            else if(islower (s[i]) && islower (k))
            {
                int cipher2 = s[i]-'a';
                int d = (cipher2 + k) % 26;
                printf("%c", d+'a');
            }

            //PRint characters
            else
            {
                printf("%c", s[i]);
            }
            // PASTED CODE ENDS HERE JUST IN CASE

        }
                //print space after i for loop
                printf("\n");
                return 0;
}

I feel like I am really close but I am stuck on this last part I am putting in the formula for k but it's not outputting the correct cipher. Anyone have any ideas I would really appreciate it.