How to fix iPhone 12 mini showing 'iPhone unavailable'?

Please follow the Support Article below. You'll most likely need to connect your iPhone to a computer and erase then restore and set a new passcode. But there is a small chance if you see the Forgot Passcode? option in the bottom right, you will also be able to erase it directly from the iPhone with your Apple Account credentials.

Start here: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support