All BigCommerce stores are set to private when first created. This keeps search engines from finding and listing your store in search results while it is still in development. Visitors need to have preview code to see your store.
If your store is still private, you will see this popup when editing a page in Zeno.
You can easily find this code if you view your storefront after you have logged in to BigCommerce. It shows at the top of the page.
If Zeno throws an error Preview code is not correct even though you have entered the correct one, then you can go to BigCommerce Admin, open Settings > Security & Privacy, and make sure to uncheck Enable reCAPTCHA on a storefront pre-launch login page