Correct, because Discourse is registered to http://fstdesk.com/ as seen in your certificate:
common name: fstdesk.com
Your certificate doesn’t include a subject alternate name for https://www.fstdesk.com so the webserver can’t honor that request without first issuing a certificate with a SAN including the WWW entry.
Google Chrome will handle root redirects, but only in the case of @ → www and www → @ - if you try the same URLs in Safari you will note that https://www.fstdesk.com also returns the error.
In short this isn’t an error, it wouldn’t work at all if you weren’t using Chrome, which tries to correct mistyped root domains.
Your options are:
- Reconfigure your let’s encrypt to include the subject alternate name www.fstdesk.com - this will mean changes to the configuration of your Discourse installation, guides already exist here to achieve this.
- Manage your redirects outside of Discourse
The simplest option is the second one, services such as Cloudflare can catch and redirect all traffic in this way with no Discourse configuration changes whatsoever.