Mailchimp - contact is already a list member in compliance state due to unsubscribe, bounce, or compliance review.
The PieSync Health dasboard shows the following error:
What is the problem?
Mailchimp does not allow emails to re-subscribe over the API once they have been deleted or unsubscribed.
It means the email address was removed or unsubscribed from Mailchimp at a certain point in time. Mailchimp doesn't allow a contact to be re-subscribed through the API once it has been deleted or unsubscribed.
Manually add the contact in Mailchimp again. Once this has been done, it will be kept in sync, just like your other contacts.