How to resolve 'iPhone 6s unavailable' issue?

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

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

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

You need to provide more information. Unavailable where? What are you trying to do?