How to resolve Exception #0 (Exception): Notice: Array to string conversion Mailchimp Error

It is an error of Mailchimp module just go to the below location

Magento_root/vendor/mailchimp/mc-magento2/Block/Adminhtml/System/Config/Form/Field/CustomerMap.php

On the line number 68 comment the below code

$optionExtraAttr['option_' . $this->_getMailchimpRenderer()->calcOptionHash($row->getData('mailchimp_field_id'))] =
            'selected="selected"';

Since initially your Magento store is not connected with the Mailchimp so it is not able to fetch the Mailchimp field values & that's why it is giving error.

Please refer this