Hello NeilM595,
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Typically the Unauthorized (401) response indicates that the access token or API key that you used when making your call is not valid or has expired.
V3 API - 401: Unauthorized
https://v3.developer.constantcontact.com/api_guide/glossary_responses.html#401-unauthorized
For further assistance, please feel free to email us directly/securely at webservices@constantcontact.com with the API key that you are using to make the request, the full request(s) where you are receiving the response error (including URL, headers, timestamp, and request body), and the full response body so that we can so that we can better assist you in identifying the root cause of the issue?
Please have a look and let us know if you have any other questions!
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