cs50/x

Hello All,
Long-time listener, first-time caller.
I've been studying this chat string for a few days to try to get help on Caesar. It has helped me immensely. Y'all seem to be pretty laid-back and more forgiving of ignorance than some of the people on the other forums.
I am ALMOST there. but when I run check50 i get the following errors.

:) caesar.c exists.
:) caesar.c compiles.
:( encrypts "a" as "b" using 1 as key
expected exit code 0, not 1
:( encrypts "barfoo" as "yxocll" using 23 as key
expected "ciphertext: yx...", not "ciphertext: yc..."
:( encrypts "BARFOO" as "EDUIRR" using 3 as key
expected "ciphertext: ED...", not "Cyphertext: EC..."
:( encrypts "BaRFoo" as "FeVJss" using 4 as key
expected "ciphertext: Fe...", not "Cyphertext: Fc..."
:( encrypts "barfoo" as "onesbb" using 65 as key
expected "ciphertext: on...", not "ciphertext: oc..."
:( encrypts "world, say hello!" as "iadxp, emk tqxxa!" using 12 as key
expected "ciphertext: ia...", not "ciphertext: ic..."
:) handles lack of key
:) handles non-numeric key
:) handles too many arguments

my code is as follows, thanks in advance.

int main(int argc, string argv[])
{
if(argc == 2)
{
for(int key = 0; key < strlen(argv[1]); key++)
{
if(isdigit(argv[1][key]))
{

        }
        else
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }
}
else
{
    printf("Usage ./caesar key\n");
    return 1;
}

int keynum = atoi(argv[1]);

string plntxt = get_string("Plaintext: ");

char len = strlen(plntxt);

for(int p = 0; p < len; p++)
{
    int cypher;

    if(islower(plntxt[p]))
    {
        if((cypher = (plntxt[p] + keynum - 97) % 26))
        {
            printf("cyphertext: %c", cypher + 97);
        }
    }
    else if(isupper(plntxt[p]))
    {
        if((cypher = (plntxt[p] + keynum - 65) % 26))
        {
            printf("Cyphertext: %c", cypher + 65);
        }
    }
    else if(!isalpha(plntxt[p]))
    {
        printf("%c", plntxt[p]);
    }
}
printf("\n");
return 1;

}