I installed iOS 10 on my iPhone 6s Plus on launch day. Have been using it without any problems. However, I just installed macOS Sierra on my Late 2013 27" iMac and after installing I turned off Two-Step Verification and turned on Two-Factor. Now for some reason on my iPhone I have a badge on the settings app that directs me to a "Follow Ups" section. In there it says "Finish Setting Up iCloud" with a "Continue" button under that, and the explanation "Enter the passcode from another device to access passwords, health, home, and other protected data stored in iCloud." I hit "Continue" and enter in my Apple ID password (keep in mind, this has NOT changed throughout the time I've been running iOS 10 or macOS Sierra). I hit "Sign In' and, after some loading, I'm faced with a screen that says "Enter Passcode" and "Your passcode will be used to confirm your identity when signing in to iCloud on a new device." I'm tried my apple ID password, my others passwords, etc..... I'm going to stop now because as I was typing this I realized I had not tried the passcode to get into my actual iPhone.... Voila. It worked. I'm going to post this so that if anyone else has the same issue they're able to realize their stupidity without having to type out as many characters...... Goodbye.