acf-featured-image.php

<?php

/**
 * Set "Featured Image" from ACF field
 */
add_action('acf/save_post', function($post_id) {
  $value = get_field('video_cover_image', $post_id);

  if($value != ''){
    add_post_meta($post_id, '_thumbnail_id', $value);
  }

  return $value;
});