I've tried on two new Pi's, so it's not a hardware issue.
Per the instructions, I flashed the Pi EEPROM for USB boot using the official Pi imager. I loaded the Channels custom OS image using the same Pi imager on a brand-new Toshiba 2gb USB3 HDD (three times just to make sure and it verified correctly all three times). And yes, the SD card I used to flash the EEPROM was removed.
The red and green lights start up solid, but the green light goes out after a few seconds (it's supposed to stay on solid green). The light on the HDD goes on and I hear it spinning at first. Then it goes off. It's plugged into my router directly, but I can't reach it via http://dvr-server.
UPDATE: I tried using a USB C hub with my laptop charger attached. The drive is attached to the hub, not directly to the Pi. The green light now goes on solid. I thought the USB C on the Pi was for power only. However still not able to reach the server via http://dvr-server.
Anyone have any idea why this is not working?