This seems to be a safety feature, to prevent the mower from rolling when unattended. I don't get any flashing icon or button after taking out the key; the LCD is off, and the "Set Button" (with gears) is lit blue.
The only way I have found to unlock the wheels is to pop up all the batteries (press each battery's release button to disengage it). This method is mentioned briefly under "Limp-Home Mode" at the bottom of page 17 in my PDF manual (undated), or the second NOTICE at the top of page 18 in my printed manual (8/30/2021).
Of course to charge you have to push the batteries back down, so it's not as convenient as might be hoped. I usually recharge before shifting the mower to its parking position, and leave the batteries disengaged until the next time I mow.
-- George
