WordPress creating a compund query or using data from one post type to access another

As per WP documentation,

Using the WP_Meta_Query::parse_query_vars( $query ) method: You can
use this method, if you want to use the simple query args(meta_key,
meta_value, meta_type, meta_compare), or if you are unsure of
the presence of meta query parameters.

Hence, I tried converting meta_key, meta_value and meta_compare in source code to wrap inside meta_query as key, value, compare.

Also, added if( $registrations->have_post() ) {

Can you check if below code snippet works for you ?

            $events = new WP_Query(Array(
                'posts_per_page' => -1,
                'post_type' => 'event'
            ));
            if ($events->have_posts()){
                while($events->have_posts()){
                    $events->the_post();
                    echo "<p>Event " . get_the_title() . '</p>';
                    $eventID = $events->get_the_ID();
                    echo $eventID;
                    $args = array(
                        'post_type' => 'registration',
                        'meta_query' => array (
                          'key' => 'event_id',
                          'value' => $eventID,
                          'compare' => '='
                        )
                    );

                    $registrations = new WP_Query($args);
                    if( $registrations->have_post() ) { 
                      while ($registrations->have_posts()){
                        $registrations->the_post();
                        echo "<p>The registrations list "  . get_the_title() . "</p>";
                      }
                    }
                }
            }

Edit 1 :

If the event_id is an ACF then you should try fetching registrations using ACF functions.

Ref: https://www.advancedcustomfields.com/resources/query-posts-custom-fields/

echo $eventID;
$registrations = get_posts(array(
   'numberposts'   => -1,
   'post_type'     => 'registration',
   'meta_query'    => array(
       array(
           'key'       => 'event_id',
           'value'     => $eventID,
           'compare'   => '=',
       ),
   ),
));

if( $registrations->have_post() ) { 
 while ($registrations->have_posts()){
   $registrations->the_post();
   echo "<p>The registrations list "  . get_the_title() . "</p>";
 }
}