So, I was able to install free letsencrypt ssl for an add on domain at siteground.
Unfortunately, after installation, web browsers kept telling visitors that the site it not safe.
I used WhyNoPadlock to check what’s wrong and I got a “Your SSL certificate does not match your domain name!” error.
I fixed it by going back to my siteground dashboard to re-install the free ssl for the addon domain. It installed successfully, without even throwing “already installed” error.
Thereafter, I used WhyNoPadlock to confirm and all errors cleared!