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

If the issue persists, contact Apple Support directly, using this link:--> Contact - Official Apple Support

Regards

Giulio

Are you using an Apple Smart (folio) keyboard?

This alert 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 is not repairable. The only remedy is replacement of the keyboard.