How old is your Apple ID?
As per Apple's support document on Two-Factor Authentication:
Two-factor authentication for Apple ID - Apple Support
Can I turn off two-factor authentication after I’ve turned it on?
You can't turn off two-factor authentication for some accounts created in iOS 10.3 or macOS Sierra 10.12.4 and later. If you created your Apple ID in an earlier version of iOS or macOS, you can turn off two-factor authentication.
If you have an account created on earlier versions of iOS or macOS, what happens when you try to turn it off? Are you not seeing the option? Where are you looking for it exactly?
Provide some details of what you have done, and what happened, so someone can help.