turn off Two-step verification for Apple ID

It can't be turned off.

See Two-factor authentication for Apple ID - Apple Support

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.

2 Step Verification or 2 Factor Authentication?

2 Step Verification 

How do I turn off two-step verification?

Sign in to your Apple ID account page.     

In the Security section, Click Edit.

Click Turn Off two-step verification.

Create new security questions and verify your date of birth.

You'll get an email confirming that two-step verification for your Apple ID is off.

Apple ID - Two Step Verifications FAQ.  *