this unit has an electric brake system on it, it will not release unless you have a 24v power source to energize the brake to make it release.
I did not see any reference in the manual for a manual release anywhere.
Some of these brake units have certain screws you can adjust to make it manually release but like i said, the manual does not mention this anywhere.
Not knowing the specific type of brake unit on the machine, my only suggestion is to get power to the unit so it will be drivable, or use a fork lift and pick it up and carry it.
I don't think this unit was intended to be able to roll freely because of the drive system on it, it serves two purposes, it drives the unit and also steers the unit using the drive motors. The only way to make it roll freely is to disengage the brake units on the motors. That signal comes from the electronic drive control module. Since there is no way to steer the unit without motor control it could roll off in any direction with no control in the steering. I think that's why the mfg does not give a way to disengage the brake easily.
the manual only speaks of using the fork pockets in the frame as a means of moving the unit (transporting) without driving it.