PlayerGui is not a valid member of DataModel?

script.Parent.ClickPart.ClickDetector.MouseClick:Connect(function()
	script.Parent.Parent = game.ServerStorage.PlayerINV
	if game.ServerStorage.PlayerINV:WaitForChild("SparkPlug") then
		game.PlayerGui.Pickup.BG.Visible = true
	end
end)

Hello.

PlayerGui is not a service, but a child of a player. In this case, you can get the player from the parameter of MouseClick, and then just indexing PlayerGui.

Here's your fixed script:

script.Parent.ClickPart.ClickDetector.MouseClick:Connect(function(player)
    script.Parent.Parent = game.ServerStorage.PlayerINV
    if game.ServerStorage.PlayerINV:WaitForChild("SparkPlug") then
        player.PlayerGui.Pickup.BG.Visible = true
    end
end)

I hope this helps. Cheers!