🇦🇺Australia @christopher.nolden

I had the same issue on a non-Composer site that utilizes Ludwig. I found that the ludwig.json file was not reflecting the new mailchimp-api-php library version requirement. Replacing the contents of the ludwig.json file with the below values allowed the new version to be installed and fixed the error for me.

{
  "require": {
    "thinkshout/mailchimp-api-php": {
      "version": "v3.0.0-rc4",
      "url": "https://github.com/thinkshout/mailchimp-api-php/archive/v3.0.0-rc4.zip"
    }
  }
}