iPhone is unavailable. What should I do?

Please follow the instructions in this Apple Support document:

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

The instructions say to tap on Forgot Passcode (lower right of the screen) Did you do this? If this is not working.......

Check further down in the instructions........and you will see this step:

If you can't follow these steps, you can use a computer to reset your device.

So, you may need to use a computer to reset your phone.