When several users reported they tried to update and got stuck in the TWRP recovery in the process, this here helped, would be worth a try in this case too, I guess …
This can be a false alarm by the TWRP recovery. Why it would do that sometimes is unknown at this point, but there’s no harm in rebooting nonetheless.
Best case: False alarm, phone boots.
Worst case: Message is correct, phone will not boot, nothing will be changed.