How to Detect Previous Placement???

This is a good question, and I'm going to provide my solution. My solution might not be the best, and if anyone reading this has a better solution, I would like to see it.

I would suggest storing the values inside a table as you create them, and check using a function if the new random Vector3 is close to that. If it is, create a new random vector.

First, I'm going to create a function that returns a random vector.

local MaxSizeX = 200
local MaxSizeY = 50
local MaxSizeZ = 200

local function RandomVector3()
    return Vector3.new(
    math.random(-MaxSizeX, MaxSizeX),
    math.random(0, MaxSizeY),
    math.random(-MaxSizeZ, MaxSizeZ),
)
end

You could construct this however you want to fit your needs.

Next I would make a function that returns a part.

local PartSize = Vector3.new(2,2,2)

local function CreatePart()
    local part = Instance.new("Part")
    part.Size = PartSize
    part.Anchored = true
    part.BrickColor = BrickColor.Random()
    return part
end

Now I'm going to create the table.

local PastPositions = {}

This will be used to store the positions.

Now we can create the main action of the script. A loop which continuously creates parts.

local MaxSizeX = 200
local MaxSizeY = 50
local MaxSizeZ = 200

local PartSize = Vector3.new(2,2,2)
local PastPositions = {}

local function RandomVector3()
    return Vector3.new(
    math.random(-MaxSizeX, MaxSizeX),
    math.random(0, MaxSizeY),
    math.random(-MaxSizeZ, MaxSizeZ),
)
end

local function CreatePart()
    local part = Instance.new("Part")
    part.Size = PartSize
    part.Anchored = true
    part.BrickColor = BrickColor.Random()
    return part
end

while true do
    local part = CreatePart()
    part.CFrame = CFrame.new(RandomVector3())
    part.Parent = workspace
    wait()
end

This creates the parts in workspace. This is probably about what you have.

Now we need to save the part's Vector3 inside the tables when it's created.

while true do
    local part = CreatePart()

    local NewVector = RandomVector3()
    PastPositions[#PastPositions+1] = NewVector

    part.CFrame = CFrame.new(RandomVector3())
    part.Parent = workspace
    wait()
end

Now we don't want to save the new Position if the Position already exists. So now we need to create a function that checks if the positions already exists in the table.

--// Returns true if vector is already in table, else returns false.
local function CheckTableForPosition(TheVector)
    for _, Vectors in pairs(PastPositions) do
        if Vectors == TheVector then
            return true
        end
    end
    return false
end

Now we can combine this with out main code.

while true do
    local part = CreatePart()
    local NewVector = RandomVector3()

    while CheckTableForPosition(NewVector) do
        NewVector = RandomVector3()
    end

    PastPositions[#PastPositions+1] = NewVector
    part.CFrame = CFrame.new(NewVector)
    part.Parent = workspace
    wait()
end

Now this will make sure it doesn't make a part with the EXACT same position, but we probably want a little space as well. The changes are below.

--// This is the closest the parts can be together.
local ClosestDistance = 5

--// Returns true if vector is already in table, else returns false.
local function CheckTableForPosition(TheVector)
    for _, Vectors in pairs(PastPositions) do

        local magnitude = (Vectors - TheVector).Magnitude
        if magnitude < ClosestDistance then
            return true
        end

    end
    return false
end

This uses magnitude to check how close the vectors are.

My final example script would look as such,

local MaxSizeX = 200
local MaxSizeY = 50
local MaxSizeZ = 200

local PartSize = Vector3.new(2,2,2)
local PastPositions = {}

local ClosestDistance = 5

--// Returns true if vector is already in table, else returns false.
local function CheckTableForPosition(TheVector)
    for _, Vectors in pairs(PastPositions) do
        local magnitude = (Vectors - TheVector).Magnitude
        if magnitude <= ClosestDistance then
            return true
        end
    end
    return false
end

local function RandomVector3()
    return Vector3.new(
    math.random(-MaxSizeX, MaxSizeX),
    math.random(0, MaxSizeY),
    math.random(-MaxSizeZ, MaxSizeZ),
)
end

local function CreatePart()
    local part = Instance.new("Part")
    part.Size = PartSize
    part.Anchored = true
    part.BrickColor = BrickColor.Random()
    return part
end

while true do
    local part = CreatePart()
    local NewVector = RandomVector3()

    while CheckTableForPosition(NewVector) do
        NewVector = RandomVector3()
    end

    PastPositions[#PastPositions+1] = NewVector
    part.CFrame = CFrame.new(NewVector)
    part.Parent = workspace
    wait()
end

Final Note

This is not meant to be copy and pasted. This is only a walkthrough of the thought process I went through to come up with a working solution.

There are probably errors, but the main idea of the script should do fine.

Thanks for reading!

function init(xpos, zpos)

local Block = Instance.new("Part", workspace) --[Declaramos la pieza]--
Block.Name = "Block"
Block.Parent = workspace
Block.Anchored = true
Block.CanCollide = false

game.Workspace.Block.Size = Vector3.new(10, 0.5, 10) --[Forma plana]--
game.Workspace.Block.Position = Vector3.new(3, 0.5, 3) --[Posición]--

local Amount = Instance.new("IntValue", Block) --[Declaramos las Reservas]--

Amount.Value = math.random(1000, 100000) 

end

init(xpos, zpos)

That's what i came up with, i lacked the randomizer and something to store the occupied areas. Thanks for the answer