How do I make it so it switches teams when you join if a gamepass is owned?

The issue here is that you used PlayerOwnsAssest instead of UserOwnsGamePassAsync. Also UserOwnsGamePassAsync requires the player's id, not the player. This is why it's useful to define your services as a constant so you can see the functions for it, instead of indexing it on one line.

local MPS = game:GetService("MarketplaceService")

game.Players.PlayerAdded:Connect(function(plr)
	if MPS:UserOwnsGamePassAsync(plr.UserId,13086230) then 
		game.Players[plr.Name].TeamColor = BrickColor.new("Really blue")
		print("Player owns the gamepass!")
	else
		print("Player does not own the gamepass!")
	end
end)