two factor authentication

See the following from Two-factor authentication for Apple ID - Apple Support

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 has been more than 2 weeks since you activated it then you can't turn it off, if it has been less than 2 weeks, see the email you received on how to turn it off.