Why are you trying to ctrl-alt-del in tablet mode?? All of the things you can do in that menu can be done without CTRL-ALT-DEL.
You can lock, unlock, access task manager, sign out, switch user, and shutdown all without accessing that menu. Just curious what it is that you are trying to do so that we can find a solution to your problem.
You're on a domain that enforces Ctrl+Alt+Del to logon and unlock, right? On Surface Pro tablets, you can hold the Windows button and press the power button.
Edit: Just noticed that this is in the Surface Book forum. No idea.
You should be able to access the Ease of Access menu and turn on the on screen keyboard. If not your GPO should not disable this features for win 8+ devices as its needed when you do not have a keyboard connected if you enforce CAD mode.
If you go into device settings, then typing you can turn on the option to add the standard keyboard to available keyboards. This allows you to choose the full standard keyboard which has the ctrl-alt-del keys.