hdiutil attach /Applications/Install\ macOS\ Sierra\ Public\ Beta.app/Contents/SharedSupport/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_apphdiutil create -o /tmp/Sierra.cdr -size 7316m -layout SPUD -fs HFS+Jhdiutil attach /tmp/Sierra.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/install_buildasr restore -source /Volumes/install_app/BaseSystem.dmg -target /Volumes/install_build -noprompt -noverify -eraserm /Volumes/OS\ X\ Base\ System/System/Installation/Packagescp -rp /Volumes/install_app/Packages /Volumes/OS\ X\ Base\ System/System/Installation/cp -rp /Volumes/install_app/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System/BaseSystem.chunklistcp -rp /Volumes/install_app/BaseSystem.dmg /Volumes/OS\ X\ Base\ System/BaseSystem.dmghdiutil detach /Volumes/install_apphdiutil detach /Volumes/OS\ X\ Base\ System/hdiutil convert /tmp/Sierra.cdr.dmg -format UDTO -o /tmp/Sierra.isomv /tmp/Sierra.iso.cdr ~/Desktop/Sierra.iso
- Create a new El Capitan (there is no option for Sierra at the moment) machine. Leave everything at default
- Boot the virtual machine from the Sierra.iso you created, which you'll find on your desktop. I had to mash F12 while the machine was starting up and then choose 'EFI CD/DVD' in the boot manager. YMMV.
- The install process should be familiar to you. If not, you probably shouldn't be playing with a beta OS ;)
- Grab a coffee, the 15 minute estimation the installer gives you is a complete work of fiction. My install took over an hour.
- Once installed, reboot and complete the new user process. Skip the AppleID login bit, it'll fail on a VirtualBox.
- Shut down the virtual machine and quit VirtualBox.

Hi there,
The problem I guess is in macOS Sierra installer 10.12.4
I used the above technique with previous version of installer 10.12.3 and works with no problems. Then you can just download and install combo update for latest one, which is 10.12.5 these days and thats it.