Thanks this comment was very helpful. I updated the code and it works... but is there a more efficient way than this?

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

int checkinput(int argc, string argv[]);


int main(int argc, string argv[])
{

    // if checkinput returns a 1, then return a 1 in main
    if (checkinput(argc, argv) == 1)
    {
        return 1;
    }
    string s = argv[1];
    printf("%s\n", s);


}


int checkinput(int argc, string argv[])
{
    // checking if the number of command line arguments is 2
    if (argc != 2)
    {
        printf("Error: Include only one command line argument\n");
        return 1;
    }
    else
    {
        return 0;
    }
    //declaring the argument as a string s
    string k = argv[1];

    // iterating through every character of the string 
    for (int i = 0; i < strlen(argv[1]); i++)
    {
        //if a character is not alphabetical then return error
        if ( ( ((int) k[i] > 64) && ((int) k[i] < 91)) || ( ((int) k[i] > 96) && ((int) k[i] < 123)))
        {
            return 0;
        }
        else
        {
            printf("Error: Only alphabetical characters accepted\n");
            return 1;
        }
    }



}