Is it prefixed by [Report Only]? If so you can ignore this kind of error. That said you won’t be able to access admin APIs from the storefront without providing an API key, which isn’t a good idea to put in the theme as it would be publicly accessible. These APIs are for apps generally, not storefront.
What are you trying to achieve? There might be another way.
How are you authenticating your request securely to the Admin API from checkout.liquid? I hope you’ve not exposed the key details for all to see in that file.