Attempt to call a nil value?

Can anyone tell me why this SurfaceGui tool giver gives me this error?

Workspace.ClickPart.SurfaceGui.Button.:2: attempt to index local 'playerWhoClicked' (a nil value)

Here is the code:

function givetool(playerWhoClicked)
    if playerWhoClicked:GetRoleInGroup(1) >= 6 and playerWhoClicked.TeamColor == BrickColor.new("Brown") then
        local toolclone = game.ServerStorage.TestTool:Clone()
        testtool.Parent = playerWhoClicked.Backpack
    else
        print("Unauthorized request!")
    end
end

script.Parent.MouseButton1Click:connect(givetool)