I'm experencing this same issue that was reported in this post
"Norton Security Service keeps turning itself off on Android 8" Posted: 07-May-2019"
My phone is a tracfone Alcatel Model TCL LX A502DL Android version 8.1.0 and I'm using the NMS app - version 22.214.171.12450 downloaded via my Norton 360 with LifeLock Select subscription. I have NMS set to "not optimized" and under Battery Management I have both "Standby intelligent power saving" and " Battery saver" set to off. And under Device Administrators - "Norton Mobile Security" is checked.
In my case, after a period of time when I open the app I see this:
> Accessibility >Downloaded services > Norton Security Service is OFF
I can reset it to an all green state by following the "setup" procedure and all seems well. But after a period of time - it turns off again.
I also noted that if I open the app , leave it on the screen and let the phone display time out and go dark - it'll stay "set-up" indefinitely. But if I open the app and press the home screen icon and the app is left only on the recent screen - it'll eventually return to this "Setup-Required" state.
I tried the "accepted solution" several times and it did not help me. I also completely remove the app and re-downloaded it with the same results.