Thank you for reaching out to Constant Contact's API Support.
In August 2018 we announced that our API Key provider (Mashery) would no longer be supporting TLS 1.0/1.1 due to security issues. The official support of TLS 1.0/1.1 stopped in September of 2018, and as of 3/5/19 at around 10am EST our provider began to actively block any secure connection that was not TLS 1.2. Based on the error message you are seeing, this is probably the cause of this issue.
Because each web server and application is different, I'm not able to provide you with detailed steps on how to ensure everything on your server is up to date. At a high level, you will want to ensure that the following platforms and connections are only using TLS v1.2:
- Web Server your application is hosted on - Internet Information Services (IIS) - .NET Framework - eCommerce Application - Web services involved in your process
If you are the application developer I would recommend reaching out to your server hosting provider for detailed steps on how to check or upgrade your TLS compatibility. If you are not the developer, I would recommend reaching out to the developer of the integration for assistance.