Hey @RubberDuckers,
The checkout shortcode works in my test site both in the builder and in the frontend.


This means that it’s highly likely that something in your WordPress setup is causing the issue. It could be a custom code in your child theme if you have made customization. It could also be a third party plugin conflict.
Would you mind switching to the parent theme then testing again if the issue persists? If the issue persists, please 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.
Please also enable debugging and error logging in your site. You can follow the instruction at https://codex.wordpress.org/Debugging_in_WordPress. Specifically look at the example configuration under Example wp-config.php for Debugging.
You need to look at the error logs after that maybe we could get information what specifically is causing the issue.
Thanks.