Hi，
The “iPhone Unavailable” screen appears when you entered too many failed screen passcode attempts（Five Times）. There’s only one thing you can do: Erase the iPhone . But you’ll lose all your data.
If the locked iPhone is running iOS 15.2 and above, you can erase the device directly from the iPhone's lock screen without a computer.
# If possible, continue to enter your device password until the "Erase iPhone" option is displayed in the bottom corner of the screen.
# Tap "Erase iPhone" and then tap "Erase iPhone" again to confirm.
# Enter your Apple ID password to log out of your Apple ID on the locked device.
# Wait for your iPhone to be completely erased and restarted and your iPhone will come back to life.
In addition, you can also use iTunes, iCloud, or Find My iPhone App to fix iPhone stuck on the unavailable screen，[link|https://www.isunshare.com/iphone-ipad/iphone-is-unavailable-how-to-unlock-it.html|this guide|new_window=true] may contain the steps to assist you with your concern.
Hope all of these can help you solve the issue.