iPhone unavailable? How do I fix

See the following instructions from Apple for information on "iPhone Unavailable" How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

including the following from that support article:

From your device's Lock Screen, try entering your passcode until the "[Device] Unavailable" screen appears and your device asks you to try again later. When you're able to, continue trying to enter your passcode until the "Security Lockout" screen appears and you get the Erase [Device] option in the bottom corner of the screen.