Argument 1 missing or nil error, why is it happening?

I wrote this script, as an attempt to make one of my AI's chat, when clicked. I used a remote function to make it client sided, instead of serversided, and I keep getting an error from the click script saying 'Argument 1 missing or nil' Here's the script:

script.Parent.MouseClick:Connect(function(player)
    if player:WaitForChild("PlayerGui").StoreGUI.Enabled == true then
        player.PlayerGui.StoreGUI.Enabled = false
    else
        script.Speak:FireClient()
        wait(8)
        player.PlayerGui.StoreGUI.Enabled = true
    end
end)