"At no time CAN YOU come into my office without knocking first" is correct. In real life, if you run into a question like this, see if you can sidestep the problem by using a different wording. "Never come into my office without knocking first" is correct, and means the same thing. "Always knock before you come into my office" means almost exactly the same thing and feels a little more friendly because it it is phrased in a positive way.

You have to perhaps understand it from a different point of view. At no time = never Never can you ………(action) You may be tempted to say YOU CAN but in this case it just have to be memorised the other way around. We use it as a very strong and explicit instructions/ statement. Hope this helps

Can you