There could be multiple reasons causing this issue. You might want to try the below steps and see if it helps.
Also,If you are trying to run a test where the confirmation page is the same as the initial landing page. The conversion trigger is simply a page refresh and is not loading properly and there is an error.
Use mboxUpdate vs. mboxCreate because the offer sometimes pulls the code to the page after it has rendered.
If you are not sure on how to use dynamic mboxes, please refer to the dynamic mboxes help guide link below :
The orderConfirmPage mbox records details about your product, permitting reports on sales data, orders, and recommendations.
You can also pass order information to any mbox (not just the orderConfirmPage mbox). You can also pass order information to multiple mboxes within the same campaign.