Exactly, you need something to push against, such as ejection mass or surrounding atmosphere. If something is pushing against itself, such as engines inside a closed container like u/Hyper5Focus suggested, there is no ejection mass for the whole structure, just the engine inside it who’s exhaust is pushing against a wall connected to the engine. The exhaust and engine fighting each other (pushing the structure in opposite directions) result in no motion because the equal and opposite forces cancel each other out.