We've just checked the code, it works fine. 

Please make sure the code you've entered is correct and the code is pasted in the current active theme functions.php file. 

 Here is an instruction on How to Edit Your Theme’s Functions.php File Without Killing Your Site:

https://www.nosegraze.com/edit-functions-php/

i think i like the embed addon and will purchase it soon. But at the main time, i want to use the alternate option you provided with the Code but it seems not working for me

I added it with my snippet plugin, deleted all caches, pressed ctrl+f5 in front end. I then login as registered user, copied a Youtube link and pasted as a comment, it just appeared as I pasted. what did i miss please

add_filter('wpforo_content_after', 'wpforo_custom_video_embed', 10);
function wpforo_custom_video_embed( $content ){
    $paterns = array();
    $paterns[] = "/<a+>\s**youtube.com\/watch\?v=(+)(*)/i";
    $paterns[] = "/<a+>\s**youtu.be\/(+)(*)/i";
    $content = preg_replace($paterns, "<iframe width=\"420\" height=\"280\" src=\"//www.youtube.com/embed/$1\" frameborder=\"0\" allowfullscreen></iframe>", $content);
    return $content;
}

Also, check out this paid addon:

https://gvectors.com/product/wpforo-embeds/