Adding a button to wp/aution plugin

You have added asterisk after the code.
Here is the perfect code.

add_action( 'adifier_single_product_description', 'extra_button_on_product_page', 9 );
function extra_button_on_product_page() {
    global $post, $product;
    if ( has_term( 'accessories1-3d-design', 'product_cat' ) ) {
        echo '<a class="button" href="www.test.com">Extra Button</a>';

    add_action( 'adifier_3d-design_description', 'extra_button_on_product_category_description', 9 );
    function extra_button_on_product_category_description() {
        if ( is_product_category('accessories1') ) {
            echo '<a class="button" href="www.test.com">Extra Button</a>';
    }
}

Maybe this is the reason you are facing problem.