Please kindly make sure that you do not use the old version of the extension. The new extension has the name of Email Forms and can be installed from Pro > Overview page. For more information:
The other point is that the warning you see should not prevent the plugin to work. This shows there should be a problem with settings somehow as the message you see is not a PHP Error and that is a PHP Warning. For more information about the PHP Warnings and the difference with errors kindly read this article:
I double checked the case in my installation and I have no problem integrating to Mailchimp. This shows a factor in your environment might cause this warning.
Make sure that your hosting service provider has the latest compatible version of the PHP it should be 5.6+ or 7.1+
Ensure everything is up to date according to our version compatibility list here. Please follow the best practices when updating your theme and plugins. Click here for more information.
Also, test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.