Unknown sources under Settings> Security is grayed out and cannot be selected when the device is provisioned using a QR code.
A popup that says “Action not allowed” will be seen on the screen if you try to select the option. The option is accessible only when the device is disarmed.
To disarm the device, tap 5 times in the bottom right corner of the screen, while a campaign is playing, and enter the security PIN.
Display for Android 7 - Add-on Guide | RTF-SC-0196 | Rev. 4