ibestme on "[Plugin: WooCommerce] Click add to cart,it always say "Please choose product options…"

Hi, in our variable product page, after click "add to cart" the error “Please Choose Product Options” keeps popping up. We have set an default color, so it has choosed.
Can you please help on fixing this for us?

I'm using the wordpress, Avada theme with woocommerce to build the product page.
Here is an example link of the error: http://www.ibestme.com/product/bluedio-bs-3-camel-portable-wireless-bluetooth-speaker-with-mic-for-calls-3d-surround-sound/

Thanks

https://wordpress.org/plugins/woocommerce/