Can someone help with connecting client to server using FireServer()?

I am trying to make a combat system,and have heard that the best way to do so is by making the server deal the damage/magnitude while the client does the animations, sounds, etc. However, when I try to use :FireServer() with it to connect the client to the server, it doesn't work. The error I always receive is "51: attempt to index upvalue 'ServerCombat' (a nil value)"

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local ServerCombat = ReplicatedStorage:WaitForChild("ServerCombat", 2)
local UserInputService = game:GetService("UserInputService")

-- A counter to help decide which animation to play
local swingCount = 0
local swingCount2 = 0
local currentSwingAnimation    

--Animation for the Light attack combo sequence.
UserInputService.InputBegan:connect(function(Input, IsTyping)
    if IsTyping then return end

if Input.KeyCode == Enum.KeyCode[Key] and Debounce then
    swingCount = swingCount + 1

    -- cancel any animation currently playing
    if currentSwingAnimation then
        currentSwingAnimation:Stop()
        currentSwingAnimation = nil
    end

    if swingCount == 1 then
        -- play the first animation
        Debounce = false
        local Animation = Instance.new("Animation")
        Animation.AnimationId = AnimationId1
        currentSwingAnimation = Humanoid:LoadAnimation(Animation)
        currentSwingAnimation.Looped = false
        currentSwingAnimation:Play()
        Humanoid.WalkSpeed = 0
        Humanoid.JumpPower = 0
        ServerCombat:FireServer()
        wait(.35)
        currentSwingAnimation:Stop()
        Debounce = true
        Humanoid.WalkSpeed = 16
        Humanoid.JumpPower = 50


    elseif swingCount == 2 then
        -- play the second swing animation
        Debounce = false
        local Animation = Instance.new("Animation")
        Animation.AnimationId = AnimationId2
        currentSwingAnimation = Humanoid:LoadAnimation(Animation)
        currentSwingAnimation.Looped = false
        currentSwingAnimation:Play()
        Humanoid.WalkSpeed = 0
        Humanoid.JumpPower = 0
        wait(.35)
        currentSwingAnimation:Stop()
        Debounce = true
        Humanoid.WalkSpeed = 16
        Humanoid.JumpPower = 50
        print("Working!")