You must add your user the vboxusers group by doing:

sudo usermod -G vboxusers -a <username>

Or by edditing the /etc/group file with nano, or vim, and adding your user name in front of vboxusers:

Remember, you must do this in the host, not in the guest. Then logout and login again, or reboot your computer.

Aditya suggest: The extension pack could be required,

see the comments for details.