With the latest release of Nougat, Android 7.0, we've seen that some users after a factory reset lost the ability to use the "Night mode" tile that was available during the preview. Mike Evans released an app just for that.
Earlier this week I factory reset my phone as I switched my encryption system to "File encryption" when that process was done, I quickly realised that "Night mode" was gone from my available tiles.
One of our team members pointed out of Telegram to an app by Mike Evans simply named "Night Mode Enabler".
If you're in a similar situation give this a shot.