What does "Effective UID is not root" mean for VirtualBox?
To resolve this on El Capitan when using Virtualbox versions lower than 6.x run the following from terminal:
for bin in VirtualBox VirtualBoxVM VBoxNetAdpCtl VBoxNetDHCP VBoxNetNAT VBoxHeadless; do sudo chmod u+s "/Applications/VirtualBox.app/Contents/MacOS/${bin}" done
Source: VirtualBox does not start on Mac OS X - Super User