It looks like you would like more information about the error "The requested resource could not be found."
MailChimp support gave us the following details:
This error describes a contact which was previously deleted. When a contact is deleted in MailChimp, the only way to add them back is through a hosted signup form.
This means that the contact will need to fill out a hosted signup form, and you’ll need to send it to them.
This is handled in Mailchimp. You have several options depending on how they were unsubscribed: use a signup form, manually re-enter their email address, or resend a confirmation email.
Please read this Mailchimp article for more details: Resubscribe a Contact: https://eepurl.com/dyimyD