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. If you are within your warranty period, you’ll need to contact Apple Support or your retailer.
You will find the 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/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:
Genius Bar Reservation and Apple Support Options - Apple