Hello,
I think I was able to get the mentioned error. But we have enabled the default WooCommerce theme – Storefront and tested the checkout process with that. And the same error happened, watch the video https://recordit.co/s9DaUNvdiv
It means that XStore theme is not the reason of the problem if that happens even with the Storefront. So, contact WooCommerce plugin support about this issue. Our theme is not the one that causes your problem, we can’t help you in this case sorry. We are able to help in case issue appears with our theme only, default WooCommerce issues are outside theme support and have be resolved by WooCommerce plugin support.
Regards