Program the firmware for 6UL

After I install the firmware into the NAND and rebooting the device I have got the message: “ERROR: Did not find a cmdline Flattened Device Tree
Could not find a valid device tree”. I have tried erase the flash memory by command “erase” and reinstall the firmware - the same result. My device is DIGI iMX6UL W164902075153. Firmware 8200400_A from P.1|Get%2520started|_____3. Thanks!

You have SBC PRO, you are using firmware for SBC express.
Your firmware is here:
Also your documentation is there too.