Have you tried this?
If you have a current nVidia graphics card, at the Clover boot screen, click on the "Gears" icon and add Boot Argument value "nv_disable=1" (without the quotes) until you install the nVidia web drivers
Boot into the El Capitan Installer on the UniBeast Installer USB thumb drive to begin the two part installation; at the end of Part 1, you need to reboot the newly created installation disk. (Optional) Nvidia graphics card users will again need to use the Boot Argument "nv_disable=1".