Developer options under Settings> About phone cannot be enabled when the device is provisioned using a QR code.
A popup that says “Action not allowed” will be seen on the screen when it is selected. The device must be disarmed in order for the developer options to be enabled.
To disarm the device, tap 5 times in the bottom right corner and enter the security PIN. Developer options can be enabled by going to Settings> About phone and tapping on the build number 6 times.