Well, normally, when Virtualmin creates a Virtual Server on your system, it sets up all aspects of that for you – web, email, dns, database, and so forth.
The error you’re receiving suggests that some part of that may already be setup, outside of Virtualmin (DNS in this case).
Was DNS for parent.tld setup already, outside of Virtualmin?
If so, my suggestion would be to remove that, if possible, and let Virtualmin set it up so that it’s then able to manage it as well.
Alternatively, you could tell Virtualmin not to enable the DNS feature for that domain.
Hmm, unfortunately, I’m a little confused as to your setup there… I’m not sure exactly what parent.tld being a root/master domain means in your case – normally, when Virtualmin is installed onto a system, no domains are configured on that server, except that the server has a hostname.
It sounds like on your system, at least one domain has some configuration that had been done outside of Virtualmin, which is what’s causing the problem you’re seeing. Virtualmin isn’t expecting to see any existing configuration of any domains.
If you want Virtualmin to be able to manage parent.tld – you would need to remove any portions of parent.tld that were setup outside of Virtualmin, and then use Virtualmin to create parent.tld.
Once Virtualmin is able to create parent.tld as a Virtual Server, it would then be able to manage it the same way it can manage client.tld.