Two-Factor Authentication

According to this Apple Support link (Two-factor authentication for Apple ID), if you already enabled the two-factor authentication you can no longer turn it off because certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, you can unenroll within two weeks of enrollment. Just open your enrollment confirmation email and click the link to return to your previous security settings. Keep in mind, this makes your account less secure and means that you can't use features that require higher security.

Hope it helps!