this accessory may not be supported

Looks like there’s a problem on your adapter. I have this same problem also. Here is the steps:

1. Make sure the adapter is real, not fake
2. plug the cable upside down
3. Check if you’re using the original white adapter.
4. Make sure the wire on the adapter is not exposed
5. Do not move the cable or shake it
6. Try a different adapter
7. Contact your nearest apple support for repair https://getsupport.apple.com

Reboot your iPhone: Restart your iPhone - Apple Support

Check for debris in the Lightning socket and—if any is noticed—gently remove it using a flexible and non-conductive tool.

If you do have an external power connection here but not any other external connections, try a different charging cable, as sometimes the cables and cable connectors csn become intermittent.

Do you perhaps have a Smart (folio) keyboard attached to your iPad? If so, try removing it.

The alert that you report is typically characteristic of a faulty folio keyboard. The electrical connections that pass though the flexible fabric hinges typically fracture - this usually resulting in errors and/or the on-screen notification that you see. 

The Smart folio keyboard itself is not repairable. If this is the issue, the only remedy is replacement of the keyboard.