Hello There,
The latest supported version of WooCommerce is only 3.2.3. Newer version will likely to display the message as you have stated in your original post. Which version of WooCommerce is installed in your site? Is there any other error message displayed when you are in the check out page? I have visited your site, added a product to the cart and proceed to checkout. I did not encounter the issue. All I am seeing is this:
Maybe the error will only appear when you are logged in. This could be caused by a 3rd party plugin. You could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If the issue still exist, please re install X theme and Cornerstone. There might be some files that were corrupted. You can review our update guide and see the Manual Update section to re install X.
Please let us know how it goes.