How can I turn off Two-factor authentication

If you enabled it within the last two weeks use the enrolment email you got when you enabled it : Two-factor authentication for Apple ID - Apple Support

Can I turn off two-factor authentication after I’ve turned it on?

If you already use two-factor authentication, you can no longer turn it off. 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 for two weeks. 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.

If it's been more than two weeks you can't.