Help me with this script?

Simple, define BP locally:

     local items = game.Workspace:GetChildren()
 for i,k in pairs(items) do
  if k.Name == "Zombie" then
   wait(5)
            local BP = Instance.new("BodyPosition")
            BP.Name = "BoPo"
            BP.maxForce = Vector3.new(1000000, 1000000, 1000000)
            BP.position = script.Parent.Position
            BP.Parent = k.Torso
            wait(8)
            k.Torso.BoPo:remove()
  end
end

Turns out, it works just fine. I just tested it. Do you mean why does it only fetch one zombie at a time? Cause it gets them all. If so, then use coroutines:

    local items = game.Workspace:GetChildren()
 for i,k in pairs(items) do
  if k.Name == "Zombie" then
coroutine.resume(coroutine.create(function(a)
   wait(5)
            local BP = Instance.new("BodyPosition")
            BP.Name = "BoPo"
            BP.maxForce = Vector3.new(1000000, 1000000, 1000000)
            BP.position = script.Parent.Position
            BP.Parent = a.Torso
            wait(8)
            k.Torso.BoPo:remove()
end),k)
  end
end