Julie, here from Shopify Support.
I'm glad you're testing your checkout; it's always important to ensure everything is in working order before you launch.
When you received this error, did you "preview" your theme, or did you go to your website as a customer would? Also, are you able to tell me which theme you're using and whether or not you're on a paid plan? If you're able to pass along your URL, I'd be happy to take a look here on my end and see what could be going on.
In the meantime, if you haven't already come across this yet, we have a Store Launch Checklist I highly recommend reviewing as well. This checklist goes over everything you need to do prior to launching, from ensuring your shipping settings are properly configured, to coming up with a prelaunch marketing plan.
I look forward to hearing from you!
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