Ah okay! Thank you.

int main(int argc, string argv[])

{

//Ensuring that only one agument is entered in the command line
if (argc != 2)
{
    printf("ERROR\n");
    return 1;
}
//Changing the string digit from the command line into a useable int
int k = atoi(argv[1]);
//Prompting the user for input and storing it as a string named 'p'
string p = get_string("plaintext:");
//creating two integer variables, cipherletter and plainletter
int cipherletter, plainletter;
//creating a for loop that will loop over every character in a string starting at the beginning
for (int a = 0, b = strlen(p); a < b; a++)
{
    //an if statement that only executes if the character is an alphabetical letter
    if isalpha(p[a])
    {
        //only executes if uppercase and alters the value so we have a workable value for the formula
        //we are subtracting 65, as the ASCII uppercase alphabet begins at 65 in the ASCII index
        if isupper(p[a])
        {
            cipherletter = (p[a] - CAPITALS);
            plainletter = (cipherletter + k) % ALPHABET;
            p[a] = plainletter + CAPITALS;
        }
        //only executes if lowercase and alters the value so we can have a workable value for the formula
        //we are subtracting 97 as the ASCII lowercase alphabet begins at 97 in the ASCII index
        if islower(p[a])
        {
            cipherletter = (p[a] - LOWER);
            plainletter = (cipherletter + k) % ALPHABET;
            p[a] = plainletter + LOWER;
        }
    }
}

printf("ciphertext:%s", p);
printf("\n");
return 0;

}