If your Mac is not booting from USB, you may need to check the paragraph below: Ensure that you firmly connected your flash drive to the USB port. Use a different USB port if the previous one has an issue. Create a new bootable drive. Shut down and reboot your Mac, if its using an Apple Silicon processor. If your Mac uses an Intel processor, reset the SMC, NVRAM, and PRAM. Also, check whether your macOS is compatible with the macOS on your bootable installer.