Damage system only visible to the first person?

There's a thing called client-server model. The thing you experienced is different, because that's the client. Each computer's screen is different, and there are differences. The server, however, has the change affected to every computer. So you'll need to use a remote event to see your work.

Local Script

local UIS = game:GetService("UserInputService")

UIS.InputBegan:Connect(function(input, gameProcessed)
    if input.UserInputType == Enum.UserInputType.Keyboard then
        if input.KeyCode == Enum.KeyCode.Q then
            game.ReplicatedStorage.RemoteEvent:FireServer() --This fires to the server.
        end
end)

Server Script

local function attack(player)
	--You'll need to change your attack code a bit if it uses something like LocalPlayer.
end
game.ReplicatedStorage.RemoteEvent.OnServerEvent:Connect(attack)