ChromeOS not booting from USB install media

I am trying to boot Ubuntu from my Chromebook. It is a Lenovo Yoga C630. I have developer and debug mode turned on, with

crossystem dev_boot_usb=1 dev_boot_legacy=1

I also ran this from the Ctrl+Alt+F2 terminal:

enable_dev_usb_boot

I created my install media with the Chrome Recovery Utility. I ran it from the 'Use local media' option, and renamed my Ubuntu.iso to Ubuntu.bin. It successfully created the install media.

I then restarted my computer and when I got to the OS Verification is off screen, I pressed Ctrl+U. I know it's Ctrl+U because Ctrl+L is boot into BIOS, and I don't have a BIOS. But when I press Ctrl+U, my computer just beeps at me.

I read somewhere that it will beep when it thinks that the media is not valid. There is little documentation anywhere. I found one piece of helpful stuff here. If you have any questions, please feel free to comment down below.

Any help is very much appreciated!