Delete contact in MailChimp

I migrated my website from OpenCart. I noted every contact who already subscribed to my old mailchimp via my old website. (These contacts were not in mailchimp).

I configured MailChimp eCommerce automation so it synced all my date from WP to MailChimp, including users.

Now, in MailChimp, all synced users have nothing in “Email Marketing” column. I want to update some of them to Subscribed because they were subscribed in the old website.

I want to delete them and add them manually first to then, resync with WP.

When I try to delete, I receive the error :

0 members successfully deleted! 1 email was not included (already unsubscribed, did not opt in to marketing, cleaned).

What can I do ?