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

If you are using a Smart (folio) keyboard, the alert is typical 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. The only remedy is replacement of the keyboard. As you appear to be within your one-year limited warranty, you need to contact Apple Support or your retailer.

You will find the Get Support link at top-right of this page. 

Alternatively, you can initiate a support case using the excellent Apple Support App. If not already installed on your device, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

A further alternative is to visit the Genius Bar at your local Apple Store, where the technicians will be delighted to assist. When visiting the Genius Bar, a reservation is recommended:

https://www.apple.com/uk/retail/geniusbar/

Hi Bellastar001,

Thank you for using the Apple Support Communities! For this issue, we would recommend the steps in the following article that can help:

If you see an ‘Accessory may not be supported’ alert

Hope this helps!

Have a great day.