How to able autolock on iphone?

My iphone is dimming automatically. So when i'm reading something it will dim. I want to make it stop. But the problem is, i can't click on the "auto lock" option at setting. Its like locked and i have to able something at setting. What do i do T_T

Watch the video, it will help you and also you must install Leo Privacy Guard in your device if you wanna secure all your personal data, such as images and videos, on it. This app is consider to be the best and now it is on the 2nd position of US.Chart.