Mailchimp's Legacy Application

From what you have told us you might be using the legacy Salesforce integration for Mailchimp (published by Rocket Science Group).  You can verify this by going to the installed packages under setup and checking he publisher name. Mailchimp has announced that it is retiring the old integration and encouraging users to switch to our updated product.  However, you can still use their old product but you would need to contact Mailchimp directly for support.

However you can use our product if you are interested. It’s important to note that our solution is an entirely separate package, meaning there is no direct upgrade path. Instead, you install our product in your Salesforce org and set it up new.  To make the transition easier, we offer a fully featured 14-day free trial, giving you time to explore all the functionality. While trialing our product, you can keep the old integration installed if you'd like. However, please be aware that if both integrations are actively syncing data, there could be potential conflicts. We recommend that you plan your migration carefully to avoid any issues. Please see our migration guide for more information. Once comfortable with our solution, you can purchase in-app and uninstall the old integration.