add_action is not defined

Hi I am trying to add this script using your plugin instead of functions.php

/** Disable Ajax Call from WooCommerce on front page and posts*/
add_action( 'wp_enqueue_scripts', 'dequeue_woocommerce_cart_fragments', 11);
function dequeue_woocommerce_cart_fragments() {
if (is_front_page() || is_single() ) wp_dequeue_script('wc-cart-fragments');
}

I get an error add_action is not defined

Thanks for any help