You can't turn off two-factor authentication for some accounts created in iOS 10.3 or macOS Sierra 10.12.4 and later. If you created your Apple ID in an earlier version of iOS or macOS, you can turn off two-factor authentication.
Keep in mind that when you turn off two-factor authentication, your account is protected only with your password and security questions.
To turn off two-factor authentication, sign in to your Apple ID account page and click Edit in the Security section. Then click Turn Off Two-Factor Authentication. After you create new security questions and verify your date of birth, two-factor authentication will be turned off. If you don't see the option to turn off two-factor authentication on your Apple ID account page, that means you can't turn it off.