Ordinarily Nginx is installed by the install.sh script by adding the switch --bundle LEMP. The script fetches whatever Nginx package is available from an Ubuntu repository.
come from Virtualmin repos and are 1) the Webmin Nginx module and 2) the Virtualmin SSL plugin. You still need to install the web server with apt-get install nginx. If a package isn’t found you probalby just need to install a repository that stores Nginx for your Ubuntu release.
If Apache is installed also and Nginx can’t start, either remove Apache or change its ports so that Nginx can use 80 and 443.
If Nginx is installed and running but isn’t showing up in the Webmin/Servers menu, try restarting Webmin and/or refreshing installed modules.