I also flashed the SD card with the 64bit image (http://dl.clockworkpi.com/DevTerm_CM4_v0.1a_64bit.img.bz2) using Balena Etcher (balenaEtcher - Flash OS images to SD cards & USB drives), but I still get the SD card not found error.
I was told by Clockworkpi Tech to start up with batteries and USB charger plugged in, which I did - still no dice.