Make the first item as default on Woocommerce product category items list

Try the following improved code, to get “active” on the first item:

<?php
    $product_cats = get_terms([
        'taxonomy'   => 'product_cat', 
        'hide_empty' => true, 
        'parent'     => 0, 
        'fields'     => 'names' // Term names
    ]);

    foreach ( $product_cats as $key => $parent_term_name ) {
        printf( '<button class="tablinks %s" onclick="%s">%s</button>',
            $key === 0 ? esc_attr( 'active' ) : '',
            "myFunction(event,'{$parent_term_name}')",
            $parent_term_name
        );
    }
?>