What often happens with newly-released Macs is that they ship with a special build of the current MacOS (in this case 10.13.2), won't boot off the normal release version until the next minor version comes out with the kexts needed to support them. So you probably have to wait until Apple releases High Sierra 10.13.3 to create a bootable installer USB that your iMac Pro will boot from.
Alternatively, you could install MacOS onto the USB using the iMac's Recovery partition, but that will result in a full OS and not an installer.