My daughters phone says iPhone unavailable how do I fix it

It says this because she forgot her passcode and tried to enter it wrong too many times. To fix this you must have a computer with iTunes or Finder installed. First, locate your original Apple charger and plug it into your computer. Next, you place your device in recovery mode by holding down button on right and plugging your charger into it at the same time. A picture of a laptop will come on the screen. After it's in recovery mode and hooked up to computer either iTunes or Finder will pop up asking you to update or restore device, hit restore. This will factory reset your device and let you start fresh with a new pin. If Find My is enabled on device, then she will have to know her Apple ID password to get back into device. If password has been forgotten, then go to iforgot.apple.com and start account recovery to reset password.

If you forgot your iPhone passcode - Apple Support