One possible reason for receiving this error message is that your Luminate Online site is configured to use a custom secure domain.

API calls should be made using that custom secure domain as mentioned here.

For example, instead of submitting to this URL:
you should use:

If you're not sure if you have a custom secure domain, you can identify it by finding a link to any published donation form and navigating to published page.  You will be redirected to the secure donation form page and can look in the URL bar of your browser to see the secure domain in use.