Why am I getting a notice: “Accessory not supported” on my iPad?

See >>> If you see an ‘Accessory may not be supported’ alert - Apple Support

If you are using a Magic or Smart (folio) keyboard, the alert is typical of a faulty keyboard. The electrical connections that pass through the flexible fabric hinges typically fracture - this usually resulting in errors and/or the on-screen notification that you see.

To remove the error, disconnect the iPad from the keyboard.