Mailchimp plugin is used in Floris theme is 3rd party plugin https://wordpress.org/plugins/yikes-inc-easy-mailchimp-extender/
In our documentation something might be missed. You can see their video tutorial here https://www.youtube.com/watch?time_continue=59&v=NRZbW-Da1XA

and knowledge base of the plugin https://yikesplugins.com/support/knowledge-base/?utm_source=wp_plugin_repo&utm_medium=link&utm_campaign=easy_forms_for_mailchimp