Hello Luigi,

While we wait for your reply to JoomlaWorks' question, here's some general guidance on that message:

1. "Undefined variable $ytQuery" is a PHP warning (not a fatal error), so your pages should still render — it only means the code reached a point where $ytQuery was used without having been assigned a value first. In practice this happens when the tag/parser path taken doesn't involve a YouTube query, which newer PHP versions (7.x/8.x) report much more loudly than older ones did.
2. Because of that, the PHP version running on your server matters a lot: the same plugin build can be silent on PHP 5.6 and noisy on PHP 7.4/8.x. That's exactly what the JoomlaWorks reply above is asking for, so please do report it along with where the warning appears (front-end article, category blog, a specific {youtube} tag, etc.).
3. As a workaround you can lower error reporting on the site (Joomla configuration or php.ini), but that only hides the message — it doesn't fix the underlying notice.

Similar "Undefined variable / undefined index / invalid argument supplied for foreach" warnings from JoomlaWorks extensions have been reported before and were generally traced to the environment's PHP error reporting level and version rather than to broken parsing — see the related threads K2 Stats (admin) error and Simple Image Gallery Pro doesn't work .

If you can confirm the PHP version and whether a newer AllVideos release is available for your installation, we can narrow it down further.