Here’s a StackOverflow answer by me for anyone who sees this error:
SSL certificate problem: unable to get local issuer certificate
…when using the MailChimp API.
The problem is due to MC using a root certificate that’s been removed from the Mozilla certificate bundle (you’re most likely to see it on Debian systems.)
This is a safer way to work around the problem that rolling all your certificates back to 2014 or disabling SSL certificate verification entirely. You also won’t need to set cURL options or edit your php.ini.
Also covered: how to ‘ping’ the MailChimp API to check it’s working.