You're using Instance:FindFirstChild() improperly. It will return nil if the object with the name can't be found.
So if you do this:
game.Workspace:FindFirstChild("Part").Name = "Hi"
Your code will error, because you are attempting to access nil.Name.
Furthermore, you should not have a RemoteEvent that is able to increase a value that the client can just request. Because exploiters can fire remotes with any arguments they want at any time, other than the implicit player argument you can trust, you should not trust the client!
As for why your code isn't working at all, you haven't provided enough context to the situation. It could be that there's no object in game.ServerStorage with the player's name in it when the remote is fired.
2 comments
whats the code for your script that calls the JumpPower event?
just use it in a local script and change plr.Name to game.Players.LocalPlayer.Name