How do I get recovery key to unlock my iPhone?

One should have been generated for you when you set the option up originally. As explained to you then, it cannot be recovered, and as such must be kept in a safe place by you.

If you no longer have the recovery key, you may locked out of your Apple ID permanently.

Click here ➡️ How to generate a recovery key - Apple Support

Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently. With that in mind, it's important to keep your recovery key in a safe place. You might want to give a copy of your recovery key to a family member, or keep copies in more than one place. That way you always have your recovery key when you need it.