Trying to install Virtualmin GPL version on Ubuntu 11.04 desktop, but seeing error of cannot recognize the operating system? I think you need to edit the /etc/issue and make sure your eventual /etc/issue file will look like the image below. Save and quit out of /etc/issue, reinstalling Virtualmin GPL version, and I think everything will work for you this time around.
Side Note: Virtualmin GPL (or Virtualmin Pro) probably only supports LTS (Long Term Support) version of Ubuntu, therefore you have to edit the /etc/issue file to make it looks like as if Ubuntu 11.04 is Ubuntu 10.04. I’m not so sure about the effect of forcing Ubuntu 11.04 to work with Virtualmin GPL version, therefore do this at your own risk if you’re using this advice on a production environment (i.e., a server that is using for critical missions).