How do I fix my iPhone showing unavailable?

You will need to reset your device. Have a look at the steps mentioned in this related article:

If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

When you see this, it means you entered the wrong passcode enough times to make your iPhone unavailable. If you do know the correct passcode, you wait it out, then enter the correct one. But if you have forgotten your passcode, you'll have to connect your iPhone to a computer with iTunes (Windows) or Finder (Mac) and erase the phone.

See this two links:

If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support