I'm very sorry but there is not quite enough information here to properly help. Can you please be more descriptive of what you are encountering as an error and what steps we can take to replicate?
It's important to understand that when asking for help to be as descriptive as possible. This allows users to replicate and help you.
Using the limited information here though, I will try my best. It sounds like you have your VM configured to a non Fully Qualified Domain Name (FQDN) and that is the issue. Virtualmin depends on this. Can you try changing your hostname so that it's set to a FQDN?
You will want to edit
/etc/hostname to match the FQDN you use.
Then you will want to modify
/etc/hosts similarly so that it has your actual FQDN instead of localhost.localdomain.
Then you will also want to rename the droplet on the DO control panel to the same FQDN.
This should help you get things going, make sure to restart the droplet or Virtualmin to have the changes take affect.