If you used the Debian Installer, you won't have U-Boot installed. Because the Installer is generic for all supported Boards, they do not install board specific U-Boot.
You have to install U-Boot in order to get Debian working.
Two options, either build U-Boot on device after Installer has finished and flash the SD-Card with U-Boot or use the files Debian provides:
/usr/lib/u-boot/rock64-rk3328/u-boot.bin
/usr/lib/u-boot/rock64-rk3328/idbloader.img
they are part of the package "u-boot-rockchip".
You have to install U-Boot in order to get Debian working.
Two options, either build U-Boot on device after Installer has finished and flash the SD-Card with U-Boot or use the files Debian provides:
/usr/lib/u-boot/rock64-rk3328/u-boot.bin
/usr/lib/u-boot/rock64-rk3328/idbloader.img
they are part of the package "u-boot-rockchip".