@este.el.paz I've had bad luck with 16.04 on my iBook G4 - it would freeze before I ever got a chance to install it.
I've successfully booted my iBook G4 from a USB stick many times in the past. If the same does not go for you, I would suggest referring to The PowerPC Linux Wiki's
Booting From USB Guide. If you are still unable to boot from your USB stick, either the image was flashed incorrectly, or that stick is incompatible with Open Firmware. Both of which we've seen many times before.
Speaking of, how did you flash the image to your USB disk? Rufus on Windows is known to produce Open Firmware-incompatible boot disks, for example.