Two Factor Authentication

From the article below. If it has been on longer than 2 weeks, you can't turn it off. 

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, however, you can unenroll for a period of 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 a higher level of security.

Apple ID -Two-factor authentication.       

There is no such link. If you enable 2FA with your AppleID, after two weeks, it is permanently on for your AppleID. There is no option to turn it off after 14 days has elapsed.

Users do not get to change the requirements Apple has set for logging into their AppleID. Apple sets the minimum password complexity rules and can require 2Factor logins if they chose to, which they have done now.

Go to Settings. Tap your Apple ID ➙ Password & Security. Tap Turn Off Two Factor Authentication.

sayehaghazadeh wrote:

So I can not turn off 2FA?

If it has been enabled for more that 14 days, you cannot turn it off.

No. And it is mandatory with an increasing number of Apple online services accessed via your AppleID - messages in iCloud, iCloud Keychain, home kit, Apple Pay etc all require it.