iPad switches off unexpectedly

As described, your iPad battery may have a fault, or may be reaching the end of its usable life. If the battery has failed or is failing, the iPad may unexpectedly turn-off - or may not successfully boot to a running state even when connected to its Power Adapter.

Are you able to use the iPad while remaining connected to your Power Adapter? If yes, while monitoring the indicated battery state, disconnect the Power Adapter; if the indicated battery state suddenly falls, or the iPad shuts-off, this test demonstrates the iPad battery having reached the end of its usable life:

iPad Repair & Service - Apple Support

You can establish eligibility and cost of a battery replacement in your country/region here:

iPad Repair & Service - Apple Support

If your iPad model still has hardware support, you might choose to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - prior to committing to a battery replacement or other repair:

Genius Bar Reservation and Apple Support Options - Apple

For practical purposes, in lieu of servicing and returning your own iPad, Apple usually exchange qualifying models of iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for battery replacement.

If your iPad is running iPadOS 12, then it would appear that it's a first-generation iPad Air. If so it's now over eight years old, so the battery is probably failing. A device that old probably isn't worth spending the money on to get an independent service center to replace the battery.

Regards.