out of one’s depth

If you are out of your depth, you don’t have enough knowledge or experience to do what you are supposed to be doing. Imagine that you are not a very good swimmer and you are suddenly in water that is too deep for you and you feel out of control. You are out of your depth.

He was fine when he was climbing on the lower slopes of the mountain but once he reached the ice on the upper slopes, he was out of his depth.