How do I turn two-factor authentication?

See Two-factor authentication for Apple ID https://support.apple.com/en-us/HT204915 and more specifically the section entitled Can I turn off two-factor authentication after I’ve turned it on? You can't turn it off unless you recently updated your account. In this case, you can unenroll within two weeks of enrollment.

If you are in the two-week period, "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."

See Availability of two-factor authentication for Apple ID https://support.apple.com/en-us/HT205075

It has never been possible to disable Apple two-factor authentication. Not past the initial two weeks, once enabled.

Two-factor means fewer folks get conned out of their credentials, which is a big and ever-increasing issue. And the ubiquitous scams and fraudsters seeking access to our credentials are only going to get better at the scams. I suspect all of us will eventually fall for one of these scams, sooner or later. And two-factor means those scams are tougher to complete.

Per Apple: What if I use two-factor authentication on a device running older software?

If you use two-factor authentication with devices running older OS versions – like an Apple TV (2nd or 3rd generation) – you might be asked to add your six-digit verification code to the end of your password when signing in. Get your verification code from a trusted device running iOS 9 and later or OS X El Capitan and later, or have it sent to your trusted phone number. Then type your password followed by the six-digit verification code directly into the password field.

This Apple ID password and verification code input happens at the password prompt in the Apple TV Settings, when you log into your Apple ID there. Use (for instance) an iPad or iPhone, Settings > select your Apple ID settings > Passwords & Security > Get Verification Code > and now append that code at the end of your password input on your Apple TV 2nd or 3rd generation. See if that works.

If you still have the Apple TV 1st generation, I don’t think that’s going to work.

See the Apple Support article(s) below.

Apple ID -Two-factor authentication