Filter WordPress posts by custom taxonomy term with AJAX

shortcode.php

<?php
/**
 * Shortocde for displaying terms filter and results on page
 */
function vb_filter_posts_sc($atts) {

    $a = shortcode_atts( array(
        'tax'      => 'post_tag', // Taxonomy
        'terms'    => false, // Get specific taxonomy terms only
        'active'   => false, // Set active term by ID
        'per_page' => 12, // How many posts per page,
        'pager'    => 'pager' // 'pager' to use numbered pagination || 'infscr' to use infinite scroll
    ), $atts );

    $result = NULL;
    $terms  = get_terms($a['tax']);

    if (count($terms)) :
        ob_start(); ?>
            <div id="container-async" data-paged="<?php echo $a['per_page']; ?>" class="sc-ajax-filter">
                <ul class="nav-filter">
                    <li>
                        <a href="#" data-filter="<?= $terms[0]->taxonomy; ?>" data-term="all-terms" data-page="1">
                            Show All
                        </a>
                    </li>
                    <?php foreach ($terms as $term) : ?>
                        <li<?php if ($term->term_id == $a['active']) :?> class="active"<?php endif; ?>>
                            <a href="<?php echo get_term_link( $term, $term->taxonomy ); ?>" data-filter="<?php echo $term->taxonomy; ?>" data-term="<?php echo $term->slug; ?>" data-page="1">
                                <?php echo $term->name; ?>
                            </a>
                        </li>
                    <?php endforeach; ?>
                </ul>

                <div class="status"></div>
                <div class="content"></div>
                
                <?php if ( $a['pager'] == 'infscr' ) : ?>
			<nav class="pagination infscr-pager">
				<a href="#page-2" class="btn btn-primary">Load More</a>
			</nav>
		<?php endif; ?>
            </div>
        
        <?php $result = ob_get_clean();
    endif;

    return $result;
}
add_shortcode( 'ajax_filter_posts', 'vb_filter_posts_sc');

on this line, 'per_page' => 12 // How many posts per page,

we need comma after 12 like this

'per_page' => 12, // How many posts per page,

Spot on Mr. @mail2ssr , thank you