I'm using Parallels 16 trial version. Trying to install El Capitan as a guest OS. Parallels installed fine on my machine. I downloaded the El Capitan disk image from the Apple site. But when I select that manually during the installation process, I get an error "Unable to detect operating system" .
OK, I resolved this. I had to actually run the installer to get the OS app before I could install in in Parallels
