Normally only Qubes-Whonix specific questions get answered here.
I'm sure you've tried this (specifically this part):
Downloading and burning
Download the ISO (and its signature for verification) from the downloads page.
“Burn” (copy) the ISO onto a USB drive (replace /dev/sdX with your USB drive device):
$ sudo dd if=Qubes-R3.0-rc2-x86_64-LIVE.iso of=/dev/sdX
Note that you should specify the whole device, (e.g. /dev/sdc, not a single partition, e.g. /dev/sdc1).
Caution: It is very easy to misuse the dd command. If you mix up if and of or specify an incorrect device, you could accidentally overwrite your primary system drive. Please be careful!
If command line dd doesn't work, you will get a quick answer with experts over at the Qubes-Users forums:
Quite often these issues come down to particular brands/manufacturers of USB sticks being incompatible and/or attempts to writing ISOs from utilities not working i.e. must use the command line.