[EDIT] Loop working fine in test mode, but not online?

I have this script where it would loop. It works in test mode, however online, it throws an error saying PlayerGui is not a valid member of Script.

Would anyone be able to help? Thanks

for _,a in pairs (game.Players:GetChildren()) do
        if not (a.PlayerGui:findFirstChild("Main")) then

-EDIT- Here is the full script that I am using this in. It prints the line 3 but not the line 7 which means it's not passing. This is also the edited code with the loop searching for PlayerGui, before on line 6, it would give an error saying PlayerGui is not a valid member of Script. So basically, it's not finding PlayerGui in the player.

wait(5)
while wait(10) do
	print("easyJet Allocated Seating: Checking players. /Script: CheckPlayer/")
	for _,a in pairs (game.Players:GetChildren()) do
		repeat wait() until a:findFirstChild("PlayerGui")
		if not (a.PlayerGui:findFirstChild("Main")) then
			print("easyJet Allocated Seating: Given booking GUI to "..a.Name..". /Script: CheckPlayer/")
			local bookedvalue = script.Parent.Booked
   			local newvalue = bookedvalue:clone()
    		newvalue.Parent = a
			local newscript = script.Parent.AllocatedSeating:Clone()
			newscript.Parent = a.PlayerGui
			newscript.Disabled = false
			local gui = script.Parent.Main
			local newgui = gui:clone()
			for _,b in pairs (game.Workspace:GetChildren()) do
				if b:IsA("Model") then
					local asvalue = b:findFirstChild("AllocatedSeating")
					if asvalue then
						if b.Name == "Model" then
							plane = b:findFirstChild("easyJetAirbus A319")
						else
							plane = b
						end
					end
				end
			end
			newgui.Parent = a.PlayerGui
			newgui.Adornee = plane.PlaneKit.Plane.AllocatedSeatingBoard
			newgui.Enabled = true
		end
	end
end

for _,a in pairs (game.Players:GetPlayers()) do
		repeat wait() until a:findFirstChild("PlayerGui") -- Repeats waiting until a PlayerGui is found.
		if a.PlayerGui:findFirstChild("Main") then
		end
end

The trick is going to be WaitForChild(). This function yields until the given object is in, so you can write the script like so:

for _,a in pairs (game.Players:GetPlayers()) do
        a:WaitForChild("PlayerGui") -- Waits until a PlayerGui is found.
        if a.PlayerGui:FindFirstChild("Main") then
		print("True")
	else
		print("False")
        end
end

Be careful though, if the object doesn't show up or you spelled it wrong, it will perpetually yield, never moving on.

I also left in the FindFirstChild part because if you are not using CharacterAutoLoad, the objects in StarterGui won't be automatically placed into the PlayerGui until they spawn the first time (which could have led to the perpetual yield).