I want to include my style.css in function.php file?

You are calling the function wrong. You have not given your filename, just a directory.

From the codex

wp_enqueue_style( string $handle, string $src = false, array $deps = array(), string|bool|null $ver = false, string $media="all" )

so in your case, you should enqueue like so

add_action( 'wp_enqueue_scripts', 'wpse_my_style' );
function wpse_my_style(){
  wp_enqueue_style( 'my-style', get_stylesheet_directory_uri() . 'path/to/your/css' );
}    

the rest of the parameters are optional