Undefined constant “gtm4wp_compile_datalayer”

Hi, I’m trying to add custom values to datalayer, I’m using your filter example found here: https://gtm4wp.com/gtm4wp-for-developers/actions-and-filters-in-gtm4wp-for-developers

This is my code

if ( function_exists( 'WC' ) ) {
	add_filter( 'gtm4wp_compile_datalayer', 'my_filter', 20 );

	function my_filter( array $dataLayer ): array {

				$order_id = $_POST['order_id'];
				$order = wc_get_order($order_id);
				$vat_numnber = $order->get_meta('billing_invoice_type');
				$dataLayer["companyPrivate"] = $vat_numnber;
				return $dataLayer;
			
	}
}

This code return the error ‘Undefined constant “gtm4wp_compile_datalayer”‘, what am I doing wrong?

Thanks