Why does it display player argument must be a player object? [RemoteEvents]

There is a TextButton inside a ScreenGui. The TextButton is the one I am using MouseButton1Click.

--This is a script inside the 'TextButton'
local function mouseClicked(player)
    game:GetService("ReplicatedStorage").ShopBuy:FireClient(player)
end

script.Parent.MouseButton1Click:Connect(mouseClicked)

--This is a LocalScript inside the 'ScreenGui' in StarterGui
local remote = game:GetService("ReplicatedStorage"):WaitForChild("ShopBuy")
remote.OnClientEvent:Connect(function()
    script.Parent.Open.Value = true
end)

--This is a LocalScript inside the 'ScreenGui' in StarterGui. This Script will open the Gui with TweenPosition.

if script.Parent.Open.Value == true then
    script.Parent.PromptPurchase:TweenPosition(UDim2.new(0.5,-225,0.5,-150),"Out","Quint",0.1,"true")
end

Note that 'Open' is a BoolValue in the Gui. It will just notify if the Gui should open.

Okay. There is a problem from now.

--Output :
FireClient: player argument must be a Player object

There was a error so I looked up on the Roblox Wiki but still the red output won't go away.

Can anybody tell me why doesn't this script(or localscript) doesn't work and can give a fix if necessary?

Thanks for your support.