The smart AC adapter output is too low for this computer What does it mean by that?

If you are using the original power adapter, it isn't damaged, and just started getting this message, it is almost certainly the power receiver inside your laptop that has developed a bad connection with your motherboard. You can try applying some pressure to plug that you insert into the laptop and you may see the "plug" or "charging" icon replace the "battery" icon in your sys-tray because you've improved the broken connection. The laptop will need to be opened up and serviced by technician who can solder a new receiver to the mother board.

This is a design flaw on almost every laptop I've seen, other than MacBook. Apple replaced the very vulnerable, standard pin & barrel connection with magnetic coupling that will break away without damaging either laptop or power cord. You can also use a MacBook on your lap without worrying about covering up any airflow vents on the underside - the other major design flaw with practically every other laptop out there.