Bad argument #2 (string expected, got Object) ?

It's because Oggetti itself is not a string, and you can't print things that aren't strings. So you need to get Oggetti's name. Also, there are some other things were you need to use name too. Here is the script:

local ServerStorage = game:WaitForChild("ServerStorage")
local Ogetti = ServerStorage:WaitForChild("Oggetti")
local ReplicatedStorage = game:WaitForChild("ReplicatedStorage")
local Items = workspace:WaitForChild("Items")

local ServerStorage = game:GetService("ServerStorage")

local function bring(posizione,oggetto)
    print(posizione,oggetto.Name)
    local ogg = ServerStorage.Oggetti[oggetto]:Clone()
    print(ogg.Name)
end

local function Prendi()
    local s = Ogetti:GetChildren()
    local f = s[math.random(1, #s)]
    return f
end

local function spawns()
    for i,v in pairs(Items:GetChildren()) do
        if v.Name == "Oggetto" then
            local a = Prendi()
            print(v.Name,a)
            bring(v.Name,a)
        end
    end 
end

wait(game)
spawns()

I hope this helped! Edit: posizione is position right? If yes, then we don't need to use their names.