Software update

Running off of Battery only during a update of the Operating System is a very risky adventure.

The Battery may run out of Power and then what will happen.

The Installation may corrupt the installation and the user will be left with a Bigger problem

Consider it like you car telling you it is running out of gas and needs to be attended to before it just turns off

It means connect the Mac to its charging source. Updating uses substantial power it you do not want a shutdown due the complete draining of the battery during the update.