Well, here is something I have done many times.
Copy the error message, in this case
"A problem was found with your Postfix virtual maps : No map sources were found in the Postfix configuration"
Possibly add "+virtualmin" and "+ubuntu" to the end of the search term.
and paste that into Google. In this case you will want to look for search results which contain the words "virtualmin" and "ubuntu".
I did a real quick search earlier and came up with the link I gave you but you will find many other search results, one of which will probably work. You will probably just have to spend some time digging around.
I have no idea.. I am only using what I though best..
I completely understand your reasoning for going with Ubuntu. I originally did the same thing myself. And I am very sure it will work, just might not be quite as easy as other OS choices. Don't abandon Ubuntu simply because you are having some difficulties and I am not as helpful as I would like to be in this case. Many other very knowledgeable users and staff here who may offer better advice in a little while.