i want to reset my iPhone7 Plus because of error message "iPhone Unavailable"and will not start.

if it is disabled/security lockout/unavailable, refer to this support article for various methods that you can use to resolve this. --> If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support Which one you can use depends upon your equipment configuration. If you can use the iOS15.2+ method you may not need a computer.

How to reset your device if you are running iOS 15.2 or later:

Advance step: If you're using an eSIM, contact your carrier before trying these steps and ask for a QR code to set up your eSIM again.

1. From your device's Lock Screen, try to enter your passcode five times until the "[Device] Unavailable" screen appears and your device asks you to try again later.

2. When you're able to, continue to enter your passcode until the "Security Lockout" screen appears and you get the Erase [Device] option in the bottom corner of the screen.
 3. Tap Erase [Device], then tap Erase [Device] again to confirm.

4. Enter your Apple ID password to sign out of your Apple ID on your device.

5. Tap Erase [Device] to permanently delete all of your data and settings.

6. When your device restarts, follow the onscreen instructions to set up your device again, restore your data and settings from a backup, and set a new passcode.

You can only reach the "Security Lockout" screen if you have an active cellular or Wi-Fi connection and previously enabled Find My ( Find My - Official Apple Support ) on your device. After you try to enter your password ten times, your device will either erase itself or show Security Lockout and the option to Erase [Device] without a timer, depending on how you previously set up your device. 

If you can't follow the above steps (e.g., you are using an older iOS version), you can use a computer to reset your device, as described in this article --> If you forgot your iPhone passcode - Apple Support .

Read the support article for full information. You can also watch the 5 min. 16 sec. video "How to restore your iPhone if you forgot your passcode" at https://www.youtube.com/watch/7s8-zLG9ySQ

Whichever method you use will result in erasing the iPhone. You can, however, restore it from a backup as described in this support article --> Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Here are resources if you are prompted for your username and password and have forgotten them:

If you forgot your Apple ID email address or phone number - Apple Support Note that typically this is an email address you use.

If you forgot your Apple ID password - Apple Support

If you do not have a computer, use this link Find Locations to find an Apple service location, or ask a friend if you can use theirs.