I would recommend trying the following:
- Take a look at this help page141 and go through the steps in the "Reset your API connection" section. That may clear things up.
- If the problem continues, contact your web hosting company and have them verify that they can reach ConvertKit's API (the URL shown in the error message) from the machine that hosts your web site.
- If they say everything looks good on their end, reach out to ConvertKit's support team at email@example.com (or with the message bubble in the lower right corner) and they can take things from there.