How To Make Click Detector Add Coins?

You need to actually bind your add function to a .MouseClick listener to run it when the Player activates the ClickDetector Object. You're also thinking of the wrong RBXScriptSignal, ClickDetectors do not act the same as a .Touched event—no Hit parameter is returned, nor should it be filtered for... the .MouseClick signal returns the Player Object that initiated the event, so you already have immediate access.

Note:

Using a numerical ValueObject is not recommended when debouncing. It is suggested debounces should be operated via Boolean logic, so please replace your ValueObject with a BoolValue Instance

Ensure the script resides within the ClickDetector Object, and is a SeverScript Instance.

local CanClick = script.Parent:WaitForChild("CanClick")

local ClickDetector = script.Parent

local function onClick(playerClicked)
    if (playerClicked and CanClick.Value ~= false) then
        CanClick.Value = false
        local Leaderstats = playerClicked:FindFirstChild("leaderstats")
        if (Leaderstats and Leaderstats.Coins) then
            Leaderstats.Coins.Value = (Leaderstats.Coins.Value + 10)
        end
        wait(1)
        CanClick.Value = true
    end
end

ClickDetector.MouseClick:Connect(onClick)

EDIT:

This is a proper leaderstat constructor Script, please apply this:

local players = game.Players

players.PlayerAdded:Connect(function(player)
    local leaderstatFolder = Instance.new("Folder")
    leaderstatFolder.Name = "leaderstats"
    leaderstatFolder.Parent = player

    local coinsValue = Instance.new("IntValue")
    coinsValue.Name = "Coins"
    coinsValue.Value = 0
    coinsValue.Parent = leaderstatFolder
end)

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