I can’t turn off two-factor authentication

Two factor authentication is a security measure on Apple devices which cannot be turned off.

See Two-factor authentication for Apple ID:

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 within two weeks of enrollment.