hey @sultan
BigCommerce doesn't send a checkout URL to Klaviyo with the Started Checkout event, so {{ event.extra.checkout_url }} will render empty.
can you please try these things?
{{ organization.url|trim_slash }}/cart.php sends them back to their cart. Catch is it relies on their browser cookie, so it only works if they open the email on the same device and browser they abandoned on.
{{ organization.url|trim_slash }}/checkout if you'd rather skip the cart page. Same cookie caveat.
Neither is a real personalized link, so test both and see which converts better for you.
also please check the started checkout metric , click into a recent event, and look at the raw properties. That shows you exactly what your store is sending instead of guessing. Also double check your store URL under Settings > Organization, since a missing slash there quietly breaks these links.
the dynamic product block itself should still populate, since BigCommerce does send item names, images and categories. If your products are blank too, that's a different issue from the button link.
I hope this helps
cheers
Arpit