Target is a nil value with 'for in pairs', why?

You declare the variable target inside the loop's scope. So it can only be used inside it, meaning it will be nil outside of it.

To fix it, just put local target the line before the loop and remove the local keyword from the one inside the loop, like this (Also you might want to add that wait() into the loop like i did):

local Tower = script.Parent
local towerHolder = Tower.Parent
local shotAmmo = towerHolder.shotAmmo
local towerHealth = towerHolder.towerHealth
local damage = towerHolder.towerDamage
local Time = towerHolder.towerTimeShoot
local Enemies = game.Workspace.Bricks.Attackers

function Fire()
    local ammo = Instance.new("Part",shotAmmo)
    ammo.Size = Vector3.new(.63,.63,.63)
    ammo.BrickColor = BrickColor.new("Black")
    ammo.CFrame = Tower.CFrame + Vector3.new(0,7.3,0)
    ammo.CanCollide = false
    ammo.Name = "Ammo"
    ammo.TopSurface = "Smooth"
    ammo.BottomSurface = "Smooth"

    for i,v in pairs(Tower.AmmoParameters:GetChildren()) do
        v.Parent.Damage.Value = damage.Value
        v:Clone().Parent = ammo
        --ammo.Follow.Disabled = true
    end

    game.Debris:AddItem(ammo,0.5)
end

while wait(1/Time.Value) do
    if towerHealth.Value <= 0 then --Tower health has depleted, game over
        break
    end

    local towerMag = towerHolder.towerMagnitude
    for i,v in pairs(Enemies:GetChildren()) do
        if v then -- if Enemies found then
            while wait() do
                local target
                repeat wait()
                target = v:FindFirstChildOfClass("Part")

                until target ~= nil and target.Active.Value == true
                break
            end
        end
    end
end

print("Game Over")