why may mac can't work in full screen

Try to enter full screen while pushing on the green dot In the top of the screen.

If you mean that the dock takes up some screen space, you can set the dock to hide automatically via system preferences>dock and menu bar> show hide dock automatically

How are you trying to invoke full screen? On our five Macs I've never had it fail in any app or in no open app for that matter. Of course your Mac is not ours hence the question.