Local Scripts and Servers

Local scripts only affect the client, or your computer. It doesn't make any changes to the server itself, therefore no one else sees the change and the server says that your health never changes. Use a regular script instead.

Communication between Clients and Server

Use a RemoteEvent. That way, a local script and server script can communicate and share data with each other. Here's an example of using a RemoteEvent. The remote event is inside of the server script (server script = regular script) inside of ServerScriptService.

--Server Script/Regular Script
script.RemoteEvent.OnServerEvent:Connect(function(player)
    print(player.Name.." fired the server event!")
end)

Local Script:

game:GetService("ServerScriptService").Script.RemoteEvent:FireServer()

Output:

Player1 fired the server event!

UserInputService and ContextActionService

The KeyDown method should not be used. Opt for UserInputService or ContextActionService instead! The reason we use these instead of KeyDown is because KeyDown can only check for keyboard presses. UserInputService can detect touchscreen and Xbox controller inputs. ContextActionService can detect multiple keys being pressed at once and other key combinations. Since we're only using one key, we're going to use UserInputService. These services must be used in a Local Script, as the server is not allowed to detect what keys you are pressing.

local UIS = game:GetService("UserInputService")

UIS.InputBegan:Connect(function(key)
    if key.UserInputType == Enum.UserInputType.Keyboard then
        print("You're pressing the "..key.KeyCode.." key.")
    end
end)

Combining UIS and RemoteEvents

Server Script/Regular Script:

script.RemoteEvent.OnServerEvent:Connect(function(player)
    print(player.Name.." clicked the C button.")
end)

Local Script:

local UIS = game:GetService("UserInputService")
local cooldown = false

UIS.InputBegan:Connect(function(key)
    if key.UserInputType == Enum.UserInputType.Keyboard and not cooldown then
        if key.KeyCode == Enum.KeyCode.C then --When C is pressed...
            cooldown = true
            game:GetService("ServerScriptService").Script.RemoteEvent:FireServer()
            wait(2)
            cooldown = false
        end
    end
end)

Final Product

Server Script/Regular Script:

script.RemoteEvent.OnServerEvent:Connect(function(player)
    local humanoid = player.Character:FindFirstChild("Humanoid")
    if humanoid then --If humanoid exists then...
        local sparks = Instance.new("Sparkles")
        game:GetService("Debris"):AddItem(sparks, 2)
        sparks.SparkleColor = Color3.new(1,1)
        sparks.Parent = player.Character:FindFirstChild("HumanoidRootPart")
        humanoid.Health = humanoid.Health+10
    end
end)

Local Script:

local UIS = game:GetService("UserInputService")
local cooldown = false

UIS.InputBegan:Connect(function(key)
    if key.UserInputType == Enum.UserInputType.Keyboard and not cooldown then
        if key.KeyCode == Enum.KeyCode.C then --When C is pressed...
            cooldown = true
            game:GetService("ServerScriptService").Script.RemoteEvent:FireServer()
            wait(2)
            cooldown = false
        end
    end
end)

Few things to note: I use Debris instead of Destroy to remove the sparkles. AddItem essentially does the same thing as Destroy, except after a certain amount of time. I put 2 seconds in the second argument, the sparks will be destroyed in 2 seconds. I said Color3.new(1,1) instead of 255,255, 0. If you're using Color3.new, every number will be multiplied up by 255, so it's actually equal. I left the last argument blank, because if you leave it blank then it will default to 0. Use Color3.fromRGB instead of Color3.new if you are using numbers 0-255 instead of 0-1.

Hope it helps!

Hello! So there are a couple of things wrong with this script. For one mouse.KeyDown is deprecated as theking48989987 has said. But also now that all games are "fliterenabled" you will need to alot more to have the health be the same value server side.

Place a "remoteEvent" inside of replicated storage and call it "Heal_Me". A "remoteEvent" is used to have localscripts comunicate to the server and vice versa. You can learn more on remoteEvents here and learn about remoteFunctions and remoteEvents here.

So once you placed the remoteEvent, make a server script and place it in serverScriptService. After that place a local script in game.StarterPlayer.StarterPlayerScripts, or StarterGui or StarterPack, but getting into the habbit of placing localscripts in game.StarterPlayer.StarterPlayerScripts is a great practice.

But as seeing this is a tool, i would keep it where it is.

Now this script in no way is the most secure, as it only does what you want, and can be edited by the player with exploits, or even worse, everyday glitches, so be careful, and edit upon this script if you want to make it more secure.

so heres the normal script that you placed inside serverScriptService:

--Wait for user to want to heal
game.ReplicatedStorage.Heal_Me.OnServerEvent:Connect(function(o)
    o.Character.Humanoid.Health = o.Character.Humanoid.Health+10
    print(o.Name.. " is healed by 10")
end)
--Surprising, not much server side, right?

After that, edit your script and use:

local cooldown= false
local uis= game:GetService("UserInputService")

uis.InputBegan:connect(function(key)
    --The key value normally has a .KeyCode attribute. Enum.KeyCode has all keys it can be. because you want C we do Enum.KeyCode.C
    if key.KeyCode== Enum.KeyCode.C then
        --Basicly what you had in your code with the healing being the remoteEvent.
        if cooldown==false then
            cooldown = true
            local sparks = Instance.new("Sparkles")
            sparks.Parent = game.Players.LocalPlayer.Character.HumanoidRootPart
            sparks.SparkleColor = Color3.new(255, 255, 0)
            game.ReplicatedStorage.Heal_Me:FireServer()
            wait(2)
            sparks:Destroy()
            cooldown = false
        end
    end
end)

Now i didnt test this, so if it doesnt work i can always fix it. but it should work fine.