I set up a youtube link in Background (pro) but it won't play.
Is there anything else you would like to set up?

There is no description of the video in the following pages.
https://unlimited-elements.helpscoutdocs.com/article/100-background-pro

Have you seen the video? Above is the Elementator's video playback widget.
Next, select the "Unlimited Elements" widget below and in the background of the Pro function, select the video and fill in the Youtube URL.

In "Unlimited Elements", when I insert a Youtube URL into the background video, it does not play.

YouTube/Vimeo link, or link to video file (mp4 is recommended).

So I uploaded the mp4 to FTP and inserted the URL, but it doesn't play.

Is this a feature of Elemenntor, not "Unlimited Elements"?
I used "Unlimited Elements" to insert the background when creating the widget.

elementor_widget.class.php

     case UniteCreatorDialogParam::PARAM_BACKGROUND:
   
    unset($arrControl["default"]);
   
    $arrControl["name"] = $name;
    $arrControl["types"] = array('classic', 'gradient', 'video');

  Doesn't this video feature belong to "Unlimited Elements"?

In Elementor, this video playback feature works in sections.
The video button does not appear in the column or widget.

Isn't that why Unlimited Elements can't play the video even if the video link is displayed in the background?

Once you have removed the features and bugs that don't work, please implement the features that Elementor provides.