boot: prompt comes from syslinux, and syslinux is set up by the penndrive software, I don't think the procedure and software you used worked properly.
I'd suggest that you recreate the thumb drive using this answer. See also this procedure from pendrivelinux.com which seems to match.
What is supposed to happen:
BIOS is supposed to find the first sector on the pen drive, written by the procedure, and transfer control to it. This is working.
The chain of events started with that action should load the syslinux program from the DOS partition on the pen drive. This seems to be working, too.
The syslinux software is supposed to find its configuration file in the same place and it should direct it to the Ubuntu linux kernel on the pen drive, perhaps putting up a menu first. I think this is where the failure is occurring.
With the loading of the Ubuntu kernel the Ubuntu linux/gnu/debian operating system should start.
Another reference that may be helpful is here.