I try running this script, but when I do there are no errors or anything in the output so I don't know what I'm doing wrong, please help thanks.
wait(3)
local player = game.Players.LocalPlayer
local char = player.Character
if player.char.Shirt == nil then
local Shirt = Instance.new('Shirt', player.char)
Shirt.ShirtTemplate = 'http://www.roblox.com/asset/?id=144076357'
end

2 comments
https://devforum.roblox.com/t/psa-dont-use-instance-new-with-parent-argument/30296
Also, for line 5, replace whats in that conditional with
not char:FindFirstChildOfClass('Shirt')