Magic mouse 2 is not recognized
There could be several issues:


Make sure your device is turned on



  The Magic Mouse and Magic Mouse 2 have a slide switch on the bottom of the device. Slide the switch to turn on the device. On the Magic Mouse, the green LED briefly lights up. On the Magic Mouse 2, green coloring is visible beneath the switch.



Make sure your  devices are setup with your Mac


See here how to set it up.

If you charge your mouse wiring a lightning cable then you can go to Bluetooth preferences to see the charge level.


Turn Bluetooth on


Go to the Apple menu -> System preferences and then Bluetooth on.

(Source)