how do I turn off two factor authentication

If you're already using two-factor authentication with your Apple ID, you can't turn it off. If you updated to two-factor authentication inadvertently, you can turn it off within two weeks of enrollment. If you do, your account is less secure and you can't use features that require a higher level of security.

Why are you unable to do it? And where are you doing it?

The easiest way: Log in to https://appleid.apple.com and click on Security settings. And also create additional trusted numbers in case your primary device is lost or unavailable.

You can’t. It is required on all new Apple ID accounts, and all older Apple IDs that have been updated to support 2FA.