Why is this not removing the item from the players Backpack?

I have a sell GUI and create a BoolValue inside of an item when it is sold and name it "flag" and set it to true. It is removing the item from the gui and giving the player the gold, but it is not removing the item from the players Backpack. I put a print inside the script to see if it is finding the item and it is. I look inside the players Backpack and the flag is on the item, but it is not finding the flag and removing the item.

local dataStore = require(game.ServerStorage.DataStore)
local RS = game:GetService("ReplicatedStorage")
local event = RS.Remotes:WaitForChild("Sell")
event.OnServerEvent:Connect(function(plr, amount)
	plr.leaderstats.gold.Value = plr.leaderstats.gold.Value + amount
	dataStore:AddStat(plr, "gold", amount)
	    for _, item in pairs(plr.Backpack:GetChildren()) do
		   print(item)
	           if item:FindFirstChild("flag") then
	 	           print("flag found")
		           plr.Backpack.item:Remove()
     	   	    end
	    end
end)

Use :Destroy() instead of :Remove()

item is actually the object, so in your script you're actually trying to find a child of the children in the backpack called "flag"

local dataStore = require(game.ServerStorage.DataStore)
local RS = game:GetService("ReplicatedStorage")
local event = RS.Remotes:WaitForChild("Sell")
event.OnServerEvent:Connect(function(plr, amount)
    plr.leaderstats.gold.Value = plr.leaderstats.gold.Value + amount
    dataStore:AddStat(plr, "gold", amount)
        for _, item in pairs(plr.Backpack:GetChildren()) do
           print(item)
               if item.Name == "flag" then
                   print("flag found")
                   plr.Backpack.item:Destroy()
                end
        end
end)

and yes, use Destroy() instead of Remove(), hope this helps

If it's in their backpack already, then it should work. But, when a player equips an item, it moves to their character. That may be your issue. Try referencing the tool from the character.