Surprised I could not find this asked.
Is there a way to get Auto-Lock "Never" option back in IOS 9.2.1
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Actually i also experienced this. I got this solved because I read a thread regarding this. It's happening because we install some certificate to our iPhone. For me I installed google something management certificate in order to use the google sheets. You can remove it by go to General>Profile.
Hope this can help you.