@shyoei: When you head to Settings > Lock Screen, and tap on Clock Style, does a pop up appear at the bottom of the screen which allows you to select the Lock Screen or Always On Display? If not, try restarting the phone in Safe Mode, as this will disable all third party apps and help to determine if the restriction is stemming from an external source. To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
Once in Safe Mode, follow the steps above and check to see if you're able to change the Clock Style for your Lock Screen or Always On Display (if applicable). If you can, restart the device as you would normally and delete any recently installed, third party apps. Let me know how you get on.
