Hi there,
Taking a look at your site and adding your product to the Cart, I’m seeing that it does not show any shipping section in Cart or Checkout (including no errors/issues):
Link to image: https://d.pr/i/y8t5sr
Please check your product settings to make sure the product is not set as “Virtual” (if set as “Virtual”, that removes it from shipping calculations):
Link to image: https://d.pr/i/ujBpfI
Please also check to make sure your shipping zones are set correctly:
https://docs.woocommerce.com/document/setting-up-shipping-zones/
As well as your flat rate method:
https://docs.woocommerce.com/document/flat-rate-shipping/
If you are still having issues, please send screenshots of your settings for that product, as well as your list of Shipping Zones under WooCommerce > Settings > Shipping, and your Flat Rate method settings. I recommend https://snipboard.io for easily sharing screenshots – please follow the instructions on the page, then paste the URL in your reply here.