Enqueue Style Function Not Loading

Your call to wp_enqueue_style() looks good and functions.php is the right place, but your code should be wrapped inside a function attached to the wp_enqueue_scripts hook. See the Combining Enqueue Functions section of the page you referenced for a full example. Generally speaking, any code you run in WordPress should be run on some hook or filter. Otherwise the code will be run as soon as the code is read which is usually not the correct time.

function mytheme_add_theme_scripts() {
  wp_enqueue_style( 'style', get_stylesheet_uri() );
}
add_action( 'wp_enqueue_scripts', 'mytheme_add_theme_scripts' );