Have a problem boot Debian from eMMC
Used the img https://github.com/radxa-build/rock-5b/releases/download/b36/rock-5b_debian_bullseye_kde_b36.img.xz and Flash via eMMC maskrom mode.
And when try to start the SB i see blue LED without any blink. From SD card starts whthout any problem
Debian eMMC boot issue
Most likely its your power supply. Try monkas armbian image.