And when I visit the same URL, I get the error message that I have posted in the screenshot above.
If you wait a few hours, I will be able to see what you see. To understand why this happens, Google DNS propagation.
Let’s Encrypt may be seeing what I see or may be seeing what you see when it tries to validate your request for a SSL certificate. The only sure-shot way to insure that your request is successful is to wait till the DNS changes that you have made to the domains propagate and resolve everywhere, then get Virtualmin to request a SSL certificate.