If the Sierra installer is still available in your account, download it and either create a dmg/iso or use the app itself as installer source (> Create a custom virtual machine). On most Apple App Store accounts the Sierra installer seems to have vanished after High Sierra was published even if Sierra was 'purchased' in the past, so you have to log-in and check your purchase list. To install macOS 10.12 in a VM on your host you either have to use an existing macOS 10.12 installer app on your Mac or a share, download the macOS 10.12 installer app from the Apple App Store which requires the app to be listed in your purchase list or install it using the recovery partition.
