What is the error (Warning: Trying to access array offset on value of type bool in... line 13 ) and how to fix it?

Question

I'm beginner in php and wordpress and I'm trying convert html template to wordpress theme and i face this issue :

Warning: Trying to access array offset on value of type bool in E:\xampp\htdocs\consulty\wp-content\themes\consulty\template-parts\content-gallery.php on line 13

I got the php code from the link above the code for creating gallery post format in index.php page

<?php 
/**
 * -- Gallery Post Format
 * https://stackoverflow.com/questions/48535908/customizing-wordpress-gallery-post-format
 */

if ( has_post_format( 'gallery' ) ) {

?>
<div id="post-<?php the_ID(); ?>" <?php post_class('gallery'); ?>>
    <?php
        $gallery = get_post_gallery( $post, false );
        $att_ids = wp_parse_id_list( $gallery['ids'] ); //***This is line 13***
    ?>
    <div class="inner-box">
        
        <div id="post-gallery-<?php the_ID(); ?>"
            <?php post_class('news-carousel owl-carousel owl-theme owl-dots-none'); ?>>

            <?php 
            for ( $i = 0; $i < count( $gallery['src'] ); $i++ ) { 
                ?>
                <figure class="image-box">

                    <img class="" src="<?php echo esc_url( $gallery['src'][ $i ] ); ?>" />
                    
                    <span class="category"><?php the_category( ', ' ); ?></span>
                </figure>
                <?php } ?>

        </div><!-- /.news-carousel owl-carousel owl-theme owl-dots-none -->

        <div class="lower-content">
            <ul class="post-info clearfix">
                <li>
                    <i class="far fa-user"></i><a
                        href="<?php echo get_author_posts_url(get_the_author_meta('ID')); ?>"><?php the_author(); ?></a>
                </li>
                <li><i class="far fa-calendar-alt"></i><?php the_time('F jS, Y') ?></li>
                <li><i class="far fa-comment-alt"></i>
                    <?php //comments_popup_link(); 
                            comments_popup_link( 'No Comment', '1 Comment', '% Commentston', '', 'Comments Off' );
                            ?>
                </li>
                <li>
                    <i class="far fa-folder"></i>
                    <?php
                                                $categories     =   get_the_category();
                                                $separator      =   ', ';
                                                $output         =   '';

                                                if ( $categories ) {
                                                    foreach ( $categories as $category) {
                                                        $output .=  '<a href="' . get_category_link( $category->term_id ) . '">' . $category->cat_name . '</a>' . $separator;
                                                    }
                                                    echo trim($output, $separator);
                                                }
                                            ?>
                </li>
            </ul>
            <h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
            <p>
                <?php the_excerpt(); ?>
            </p>
            <div class="read-more-link">
                <a href="<?php the_permalink(); ?>">
                    <i class="fas fa-arrow-right"></i>
                    <span><?php _e('Read More'); ?></span>
                </a>
            </div>
        </div><!-- /.lower-content -->
    </div><!-- /.inner-box -->
</div>
<?php } ?>

What this mean and how to fix it?

Thank you