How to fix iPhone unavailable

(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 and newer) How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode

How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode – Apple Support (UK)

What do you mean by unavailable? Please try restating your question and explain exactly what you are trying to do. Include whatever problems you are having and any error messages.