How is shirt not a valid member of Player not working..?

Code :

script.Parent.MouseButton1Click:connect(function()
	
local plr = game.Players.LocalPlayer
local Shirt = "http://www.roblox.com/asset/?id=128133687"
local Pants = "http://www.roblox.com/asset/?id=128133700"

plr.Shirt = "http://www.roblox.com/asset/?id=128133687"
plr.Pants = "http://www.roblox.com/asset/?id=128133700"
end)

To create an object you need to use Instance.new(object's class name, it's parent)

The shirt and pants are part of the character model and not the player instance. In addition, you need to change the ShirtTemplate property of the shirt. This would like this:

script.Parent.MouseButton1Click:connect(function()
 
	local plr = game.Players.LocalPlayer
	local char = plr.Character or plr.CharacterAdded:wait()

	local Shirt = "http://www.roblox.com/asset/?id=128133687"
	local Pants = "http://www.roblox.com/asset/?id=128133700"
 	
	if char:FindFirstChild("Shirt") then
		char.Shirt.ShirtTemplate = Shirt
	else
		local newShirt = Instance.new("Shirt")
		newShirt.Parent = char
		newShirt.ShirtTemplate = Shirt
	end
	if char:FindFirstChild("Pants") then
		char.Pants.PantsTemplate = Pants
	else
		local newPants = Instance.new("Pants")
		newPants.Parent = char
		newPants.PantsTemplate = Pants
	end
end)