[Kanegasi]

It's possible the LOOT_CLOSED event is too early to use, since it fires before the last CHAT_MSG_LOOT event for everything you looted. I suggest using BAG_UPDATE_DELAYED, which fires once at the end of any bag updates. I also suggest using that filename variable instead, since the first return won't be "Mage" if your client is not English. The filename is an all-caps identifier that will always be the English name in every client. It's also possible that UnitClass doesn't return anything while addons are loading. It's not uncommon to wait for the ADDON_LOADED, PLAYER_LOGIN, or even PLAYER_ENTERING_WORLD events before requesting data.

I rearranged your code with my suggestions and cleaned it up a bit:

local UnitClass,GetContainerNumSlots,GetContainerItemInfo,PickupContainerItem,DeleteCursorItem,UseContainerItem,select
    = UnitClass,GetContainerNumSlots,GetContainerItemInfo,PickupContainerItem,DeleteCursorItem,UseContainerItem,select
 
local _, class
 
local items = { -- abandon list
 
    --food
    [4599] = true,
    [4608] = true,
 
}
 
local open = { -- open list
 
    [2744] = true,
    [5523] = true,
    [5524] = true,
    [7973] = true,
    [15874] = true,
    [20767] = true,
    [21150] = true,
 
}
 
local f = CreateFrame("frame")
f:RegisterEvent("PLAYER_LOGIN")
f:RegisterEvent("BAG_UPDATE_DELAYED")
f:SetScript("OnEvent", function(self, event, ...)
    if event == "PLAYER_LOGIN" then
        _, class = UnitClass("player")
    else
        for b = 0, 4 do
            for s = 1, GetContainerNumSlots(b) do
                local id = select(10, GetContainerItemInfo(b, s))
 
                if class == "MAGE" and items[id] then
                    PickupContainerItem(b, s)
                    DeleteCursorItem()
                    --print("clean")
                end
 
                if open[id] then
                    UseContainerItem(b, s)
                    --print("open")
                end
            end
        end
    end
end)