load jQuery UI library into my plugin php

Maybe the problem lies that you are loading the jquery on the page after the place you are using it.

Try to use a hook to load the script, and hook in on page load or some other action that is done before the page loads.

Also check the version of jquery and add a row of console.log(‘Test’) to see if it rely loads or not.

Try finding a hook on this link:
https://codex.www.ads-software.com/Plugin_API/Action_Reference

I use it in the hook ‘wp_enqueue_scripts’ and its inside a plugin so its probbably before any use. I paste the code itself to see maybe I make some other mistake:
add_action( ‘wp_enqueue_scripts’, ‘ch8lj_front_facing_pages’ );
function ch8lj_front_facing_pages() {

wp_enqueue_script(‘jquery-ui-core’);
}
Thnx