Hi Es, thanks for using Caldera Forms. This is an error message we support for all the time for the Mailchimp addon. If you are trying to get help with the Mailchimp addon, please be aware that as per WordPress.org rules, support requests relating to any of the premium add-ons that exist that work alongside Caldera Forms Free must be submitted through Priority Support on our website at calderaforms.com/support. This is a rule of WordPress.org (see https://wordpress.org/support/guidelines/#do-not-post-about-commercial-products) as well as the business model by which we are able to provide a free, fully featured form builder plugin to you and over 100,000 other WordPress users.
I didn’t see you mention Mailchimp by name, but it doesn’t make a lot of sense for you to be getting this error without that addon, so I assume it is. If this is not the case, please let me know and I can take a closer look.
Thanks for understanding, and we’ll get to the bottom of your issue in 1 business day or less in Priority Support.