I installed VirtualBox from here: https://www.virtualbox.org/wiki/Downloads on macOS 10.13.3 and I am an admin on the computer. When I try to run the application it does not start. I tried to reinstall the application, but nothing changed. What am I doing wrong?
These two commands solved my problem:
sudo chown root:admin /Applications sudo chmod o-w /Applications
I didn't have the right permissions.
Since you do get a The application “VirtualBox” can’t be opened error when trying to open it via Spotlight, my first suggestion would be to just reboot your Mac and run the VirtualBox.pkg installer again.
Once finished, try running VirtualBox again and we'll go from there if it doesn't work.