"error_message": "EmailCampaignActivity Reply To Email is null or empty, please include a valid EmailCampaignActivity Reply To Email"
Thank you for reaching out to Constant Contact's API Support.
That error indicates that you have not included a valid and verified email address in the reply-to field for the email address. If you have not already verified an email address for your account you need to do that first. Once you have verified an email address you can set that address as the reply-to address.