TWO FACTOR AUTHENTICATION PROBLEM

The two-factor authentication setting is associated with your Apple ID, and which can be accessed via various paths.

On your iPhone, selecting your Apple ID within Settings gets you to the settings associated with your Apple ID.

Among the choices, Password & Security is what you'll want to access.

That'll give you an option to change your password, and will give you the setting of two-factor associated with the Apple ID.

If you have no two-factor option shown, is this iPhone associated with a business or organization, or have profiles loaded?

What region is your Apple ID set to use it? Do you live in a region where 2FA is supported? See: https://support.apple.com/en-ie/HT205075

Contact Apple Support and explain your problem, using this link:--> Contact Apple for help with Apple ID account security - Apple Support

Regards

Giulio

Go to Settings>General>Language and Region and select your region. Do this even if it already says "United States". Go ahead and re-select it. See if that helps.

Start here:--> Two-factor authentication for Apple ID - Apple Support

Regards

Giulio