To answer your question in the topic of the thread: “Are button presses only working on the server?”… The answer is of course “No” - but you’ve already worked that out.
The thing I can’t wrap my head around is… why are you constantly launching your character on tick? I get that it only activates when you “press jump” but in that “instant” your game ticks a bunch of times. Each time you launch again and again and again… That’s probably why it “jitters” - why are you doing that?