If you have an earlier version of Server it wont work with Sierra. In keeping with Apples release cycle we anticipate macOS 1013 High Sierra will no longer receive security updates starting in January 2021. Setting up a local web server on OS X 1011 El Capitan Setting up a local web server on macOS 1012 Sierra Setting up a local web server on macOS 1013 High Sierra Setting up a local web server on macOS 1014 Mojave Setting up a local web server on macOS 1015 Catalina Setting up a local web server on macOS 11 Big Sur.
The file is compressed into a zip file that contains the virtual image.