why will studio see that a player has a GUI but not in the real game?

In studio, this script works perfectly:

game.ReplicatedStorage.spon1.OnServerEvent:Connect(function(player)
local a =	Instance.new("StringValue")
	a.Name = "spon"
	a.Value = player.Sponsor.Frame.items.a.Text
	a.Parent = game.ReplicatedStorage.Sponsored
end)

However, when I try it in the real game, it says that 'sponsor is not a vaild member of playergui', when it is. Does anyone have any idea why this is happening?

Assuming this is in a Script, you cannot access the PlayerGui through the server. You need to use a LocalScript for that. The reason it works in Studio and not in the game is that in Studio Solo Mode, the client and server aren't separated. This can allow LocalScripts to do things server scripts should be doing, and vice versa.