未捕获错误：在网址中找不到类'WP_Query' - Wordpress

i created an ajax file that pass ID in the page. and that page has a query. and i got an error. I try to know what is my error and some site said i have to define my template and require the wp-load.php, but some site said that i have to do it on function.php. and i really don't know what to do. i'm new in wordpress woocommerce.i created an ajax file that pass ID in the page