Getting collection was modified;

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C#

Hey guys, I'm getting a Collection was modified; enumeration operation may not execute. error. Everyting is working fine and saving i just get spat that error still.

public static void addItem(SocketUser user, SocketGuild guild, Item item, int amount)
{
    checkUser(user, guild);
    if (getItemList(guild).ContainsKey(item.Id)) {
        List<localItem> inventory = getPlayerInventory(user, guild);
        List<string> playerList = getInventoryList(guild);
        if (inventoryContains(item, inventory)) {
            localItem local = new localItem();
            local = getLocalItem(inventory, item);
            inventory.Remove(getLocalItem(inventory, item));
            local.amount += amount;
            inventory.Add(local);
        }
        else
        {
            localItem local = new localItem();
            local.item = item;
            local.amount += amount;
            inventory.Add(local);
        }
        foreach (string line in playerList)
        {
            if (line.Contains(user.Id.ToString()))
            {
                string newline = embedInventoryString(user.Id.ToString(), inventory);
                playerList.Remove(line);
                playerList.Add(newline);
                File.WriteAllLines("server data\\inventoryData" + guild.Id.ToString() + ".json", playerList);
                Console.WriteLine("done 2");
            }
        }
    }
}

What I have tried:

I've tried moving the File.WriteAllLines around because i was thinking it was just being modified at the wrong time, i don't see anything wrong with the code?