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.
I'm sorry for the difficulty you are experiencing regarding posting to Instagram using Social share. While the error you are experiencing when attempting to post to Instagram via Social Share does refer to an API connection, our Social Share tool utilizes Facebook’s API (not ours) for Instagram and Facebook connections.
I’m going to escalate this case to one of our Tier 2 support specialists so that they can follow up directly regarding this issue. They should be in touch within 4 business days. Otherwise, for immediate assistance, or if you’d like a callback/email from them at a certain day/time you can call our support line at 866-289-2101 and reference case # 30694565 .
Please have a look and let us know if you have any other questions!
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