It does sound like you are doing a single contact add as the error you receive is only through the single contact add. Basically, that error means that a contact is trying to be modified or created in a state that requires ACTION_BY_VISITOR (indicating the Contact asked to be subscribed). There are a few reasons that could occur, most commonly that the contact already exists, is opted out and you are attempting to opt them back in.
We would need to see the raw information about the Contact you're trying to create/update, the payloads and do some investigation to get you a more detailed answer than this. If you could email our API support team (webservices@constantcontact.com) with the details that would help.
Also wanted to point out, we do not permanently delete Contacts. If you use the "Delete" option in our web UI, the Contacts are marked as "deleted" but stay in the database forever. Thus, the API integration is likely trying to update one of those deleted Contacts and there is a disconnect in the states.
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