Ssl Error

I have installed SSL on my server and it's perfectly working. However, when I tried to press

Admin panel > security settings > Enable secure connection in the administration panel

I keep getting this error

WarningThe secure connection check has failed. Please ensure that SSL certificate is installed on your server. Check out the Knowledge base article for more details.

How do I fix this?

I have installed this on a sub domain http://subdomain.domain.com