Don’t confuse sub servers with subdomains. Yes, Virtualmin does offer sub servers but you don’t need to use sub servers to host your subdomains.
You should use Virtualmin’s top level virtual server for your domain and then create another top level virtual server for your subdomain. That’s what you should do unless you have a very specific reason to use sub servers.
If you wish to use a sub server for your subdomain then you will create a top level virtual server for your domain (as usual) and then create a sub server under it for the subdomain.
See
And also