okay so it looks like have customized your mailchimp list to require some merge fields. That will be an issue:
API error occurred during signup: 400: Your merge fields were invalid.
Your list cannot have any required fields other than the “email” address. A required filed will make the API call fail if the data is not passed. Please edit your MailChimp list in question and make sure none of the fields are required there. Our plugin will add the user to the list with the email and name (if billing address collection is enabled).