See https://searchwp.com/docs/kb/searching-woocommerce-orders/ for full walkthrough to integrate SearchWP and WooCommerce Order searching

functions.php

<?php

/**
 * Let SearchWP index WooCommerce Order Notes
 */
function my_searchwp_indexer_pre_get_comments() {
	remove_filter( 'comments_clauses', array( 'WC_Comments', 'exclude_order_comments' ) );
}

add_action( 'searchwp_indexer_pre_get_comments', 'my_searchwp_indexer_pre_get_comments' );