Two-factor authentication can be turned off up to two weeks after enabling it. If it has remained enabled for over this period of time, you are locked into using it (for your account's safety, as per Apple.)
It is a useful feature which adds an extra layer of security for your account. Also, as per Apple, even if you do not wish to use it, certain features in iOS and macOS software require this extra level of security.
From the Apple Support document on Two-factor authentication, Two-factor authentication for Apple ID:
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, you can unenroll for 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 higher security.