Guzzle httpClient Drupal Core throwing cURL error 28 Operation timed out
As @cilefen suggested in his comments my problem was related to the timeout value that was being set in the calling code rather then an error with an undefined constant. I had fixed the error in my particular situation by adjusting the timeout value in my custom code that was making the API call with the following (600 is an example).

$request = $client-&gt;post($url, [
  'timeout' =&gt; 600,
  'json' =&gt; [array(...)],
]);