Why doesn't this gamepass script work? I have tried everything.

This is the script. I don't know why it doesn't work.

script.Parent.Touched:Connect(function(touch)
	if touch.Parent:FindFirstChild("Humanoid") then
	game.Players.PlayerAdded:Connect(function(player)
		local MS = game:GetService("MarketplaceService")

	if MS:UserOwnsGamePassAsync(player.userId, 16777517) == true then
			script.Parent.CanCollide = false
			script.Parent.Transparency = 0.8
		else
			MS:PromptGamePassPurchase(player, 16777517)
		end
	end)
	end
end)

Manby, you misunderstand the PlayerAdded Event.

PlayerAdded is only fired when a new player is added, doesn't has to be the player who hit the part. instead of PlayerAdded, use GetPlayerFromCharacter.

here's the fixed version of your script. Make sure it is a server script.

script.Parent.Touched:Connect(function(touch)
	if touch.Parent:FindFirstChild("Humanoid") then
		local player = game.Players:GetPlayerFromCharacter(touch.Parent)
		local MS = game:GetService("MarketplaceService")
		if MS:UserOwnsGamePassAsync(player.userId, 16777517) == true then
			script.Parent.CanCollide = false
			script.Parent.Transparency = 0.8
		else
			MS:PromptGamePassPurchase(player, 16777517)
		end
	end
end)