how to add custom reviews button in woocommerce order complete generated mail

You can overwrite woocommerce templates placing them in your theme. To overwrite e.g. email-order-details.php template, place it in yourtheme/woocommerce/email/email-order-details.php path.

You can also add action to add your custom review button.

function wpse_287685_woocommerce_email_after_order_table($order, $sent_to_admin, $plain_text, $email) {
    ?>
        <a href="#">Add review</a>
    <?php
}

add_action( 'woocommerce_email_after_order_table', 'wpse_287685_woocommerce_email_after_order_table', 10, 4 );