How to fix “iPhone Unavailable”?

You can’t open it without data loss; that’s what backups are for.

If your iPhone has iOS 15.2 or later installed, check out this link→How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support. Otherwise:

See this Apple support link→If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or watch this companion video ⬇️

After restoring the phone using one of the methods above you can restore your most recent backup.

Try these steps from this article: https://support.apple.com/en-us/HT201263

Make sure to click Update and not Restore when prompted.

Put your device in recovery mode and set it up again

1. Make sure that your Mac is up to date. If you're using iTunes, make sure you have the latest version of iTunes. 
2. On a Mac with macOS Catalina, open Finder. On a Mac with macOS Mojave or earlier, or on a PC, open iTunes. If iTunes is already open, close it, then open it again. Find out which macOS your Mac is using.
3. Keep your device connected and wait until the recovery mode screen appears:

• iPhone 8 or later, including iPhone SE (2nd generation): Press and quickly release the volume up button. Press and quickly release the volume down button. Then, press and hold the side button until you see the recovery mode screen.

• iPhone 7, iPhone 7 Plus, and iPod touch (7th generation): Press and hold the top (or side) and volume down buttons at the same time. Keep holding them until you see the recovery mode screen.

• iPhone 6s or earlier, including iPhone SE (1st generation), and iPod touch (6th generation) or earlier: Press and hold both the Home and the top (or side) buttons at the same time. Keep holding them until you see the recovery mode screen.

4. Locate your device on your computer. When you see the option to Restore or Update, choose Update. Your computer will try to reinstall the software without erasing your data. Wait while your computer downloads the software for your device. If the download takes more than 15 minutes and your device exits the recovery mode screen, let the download finish, then repeat step 3.

5. After the Update or Restore completes, set up your device.