There is a bug that makes mailchimp action is not executed.
On the file:
wp-content/plugins/yith-woocommerce-popup/includes/newsletter-integration/Mailchimp.php
Line 77 change this:
add_action( 'wp_ajax_ypop_nopriv_subscribe_mailchimp_user', array( $this, 'subscribe_mailchimp_user'));
for this:
add_action( 'wp_ajax_nopriv_ypop_subscribe_mailchimp_user', array( $this, 'subscribe_mailchimp_user'));
