My phone says iPhone unavailable and I don’t know how to reset it

If your iPhone is unaviable you will need to erase it in order to regain access to your iPhone

You will lose all data on your iPhone when you have not have access to a backup.

See this instructions on how to erase your iPhone

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.

If you can't follow these steps, you can use a computer to reset your device.

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer. 

You need to mention which part from the above article was not clear to you.

Screen Lock Passcode: By default 6 Digit

Screen Time Passcode: By default 4 Digit

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

If you enter the wrong passcode on your iPhone Lock Screen too many times, an alert lets you know that your iPhone is disabled. (Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

Follow the steps in the support document you posted from. It tells you exactly what you need to do.

If you are having an issue with those steps, then explain what that is, and someone may be able to advise.