Cannot use FaceTime HD Camera on VirtualBox Ubuntu 18.04 guest OS
Your problem seems to be that Ubuntu 18.04 doesn't come with support for the FaceTime HD camera out of the box. In addition to setting up VirtualBox correctly (which you seem to have done), you'll also need to install driver support within Ubuntu.

You can use the script given here:

https://gist.github.com/Stono/990ea9f0b3c41606c292f00382d421bf

or download the driver source code manually here:

https://github.com/patjak/bcwc_pcie

The GitHub link also has pointers to a Wiki with more information.