Virtualbox cannot be installed on MacOSX Sierra with the msg : installer: The install failed (The Installer encountered an error that caused the installation to fa...

It is blocked because of your security configuration.
On System Preference > Security and Privacy, on the right below, you see Allow button.
You need to push that button, since the access control blocking your installation.


https://apple.stackexchange.com/questions/301303/virtualbox-5-1-28-fails-to-install-on-macos-10-13-due-to-kext-security