Notify me when my Apple Pencil get low charged

Your iPad will not natively Notify you of your Pencil having low charge. You can monitor your Pencil's state of charge using the Battery Widget that can be added to a Home Screen or your Today view.

Add, edit, and remove widgets on iPad - Apple Support

While the Pencil is being used, you can also check your Pencil battery status directly from iPad settings:

Settings > Apple Pencil

While iPad doesn't natively remind you to charge your Pencil when its battery is low, you can create a Custom Automation (using native functions) that will inform you of low-charge. This external article describes how - although you may need to make appropriate adjustment to monitor your Pencil:

Create a custom battery-level alert automation