fw_printenv / fw_setenv on Armbian 5.6x does not work out of box:

# fw_printenv
Cannot parse config file '/etc/fw_env.config': No such file or directory

To solve this issue, put the following to /etc/fw_env.config:

# Device to access      offset          env size
/dev/mmcblk1            0x27400000      0x10000

IIRC Armbian 5.40- had this file in its image, not sure why the newer version hasn’t.