Nexus 6P. After every restart, phone is locked with the message: Device locked because SIM card has been changed.
Uninstalling/Reinstalling does nothing
Also, the SMS commands to lock the phone, doesnt work, you just push OK.
No need to enter a passcode to unlock. Which is pretty useless 
Thanks for the added information. Norton is looking into this. It appears to be an Android 7 issue, as other users with other devices on Android 7 are seeing the same behavior.
Update: Removing Android SIM card removal lock setting, does not stop Anti Theft 'OK' message from appearing. Just click and you're in
You're a champion. thanks for your help
OK. I'll pass this thread to my Norton contacts. They can check to see if it is a device or OS version issue.
Yep NMS is in administrator settings, everything turned on, logged in and authorised.
This is the only device I have on Android 7. Anti theft works well with all samsungs ive owned, for android versions 4 - 6
And you have the Device Administrator setting turned on from within the NMS app?
It could be something that is device specific, or some issue with Android 7. Do you have any other devices with Android 7?
Thats correct. It unlocks, and doesnt take me to number pad
So to confirm. Tapping OK does unlock the device. It does not take you to a log in screen, (number pad).?
I have the Android SIM card lock enabled. Just disabled it, restarted, and the above message didnt appear; However i tried the anti theft locate sms, and the above message appeared without the passcode option again. Just click OK to unlock
Are you sure it is the Norton SIM lock message and not the Android SIM lock feature? If you find the Android setting, ensure it is disabled.
Is it possible to get a screenshot of the message before you press OK to clear the lock? If so, instructions to post screenshots can be found here
https://community.norton.com/forums/how-post-image-forums-0
Android version is 7.1.1
I have removed Administrator, cleared app data, signed in, set up everything. After restart phone, the same thing happens: Device locked for SIM card change, press OK (no passcode needed)
I have paid version for years and always worked well with all samsung phones. Recently changed to Nexus, and it hasnt worked yet. 
What is the Android version on your device?
Have you activated the full version on the device by signing into your Norton Account.
Instead of uninstalling, try going into Android Settings - Apps and find Norton Mobile Security (NMS). Clear the app cache and app data, then restart the device. You may first have to go into NMS settings and scroll down to the Anti Theft section. turn off the Device Administrator setting. Be sure you turn this setting back on after deleting the app data.