How to fix/simplify this Placement System Script?

Me and my friend have worked together on this placement system script for a while now as a school project.

I need to make this code shorter and more efficient/clean.

The whole code is here:

wait()
local rs = game:WaitForChild("ReplicatedStorage")
local base = game.Workspace.GrassTop
local placeablePart = rs:WaitForChild("PlaceableObjects")
local Menu = game.Players.LocalPlayer.PlayerGui.PlacementSystem.ObjectMenu.ObjectList
local CurrentlyPlacing = false
local rotation = 0
local canPlace = false

local first = true

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local runService = game:GetService("RunService")

function roundAToB(a,b)
    return(math.floor((a/b)+0.5)*b)
end

local modelPreview, modelPlace

modelRotate = mouse.KeyDown:connect(function(key)
    if (key == "r") then
        rotation = rotation + 90
    end
end)

---------- OBJECTS ----------

Menu.Wall.MouseButton1Click:connect(function()
    script.SelectedPart.Value = placeablePart.Wall
    CurrentlyPlacing = true
    cloneModel = script.SelectedPart.Value:clone()
    cloneModel.Parent = game.Workspace
end)

-----------------------------

count = 0
modelPreview = runService.RenderStepped:connect(function()
    if CurrentlyPlacing == true then
        local modelCFrame = cloneModel:GetModelCFrame()
        mouse.TargetFilter = cloneModel
        local hit = mouse.Hit.p
        cloneModel:SetPrimaryPartCFrame(CFrame.new(roundAToB(hit.X,4),1.5,roundAToB(hit.Z,4)) * CFrame.Angles(0, math.rad(rotation), 0))        
        if (cloneModel.PrimaryPart.Position.X > base.Position.X-base.Size.X/2) and (cloneModel.PrimaryPart.Position.Z > base.Position.Z-base.Size.Z/2) and (cloneModel.PrimaryPart.Position.X < base.Position.X+base.Size.X/2) and (cloneModel.PrimaryPart.Position.Z < base.Position.Z+base.Size.Z/2) then     
            canPlace = true         
            cloneModel:Destroy()
            cloneModel = script.SelectedPart.Value:clone()
            cloneModel.Parent = game.Workspace
            cloneModel:SetPrimaryPartCFrame(CFrame.new(roundAToB(hit.X,4),1.5,roundAToB(hit.Z,4)) * CFrame.Angles(0, math.rad(rotation), 0))
        -- error is somewhere around here       
            for i,v in pairs(cloneModel:GetChildren()) do
                if v:IsA('BasePart') then
                    v.Transparency = 0.7
                    v.CanCollide = false
                end
            end
        else
            canPlace = false
            for i,v in pairs(cloneModel:GetChildren()) do
                if v:IsA('BasePart') then
                    v.Transparency = 1
                    v.CanCollide = false
                end
            end
        end
    else
        if not first then
            cloneModel:Destroy()
        end
    end
end)

modelPlace = mouse.Button1Down:connect(function()
    if CurrentlyPlacing == true then
        if canPlace == true then
            local new = cloneModel:clone()
            new.Parent = workspace.Tycoons.TycoonTemplate.PurchasedObjects
            for i,v in pairs(new:GetChildren()) do
                if v:IsA('BasePart') then
                    v.Transparency = 0
                    v.CanCollide = true
    --          Specific part properties below:
                elseif v.Name == "Arrow" then
                    v.CanCollide = false
                    v.Transparency = 0.7
                elseif v.Name == "Drop" then
                    v.CanCollide = false
                    v.Transparency = 0.5
                end
            end
        end
        cloneModel:Destroy()
        script.CurrentlyPlacing.Value = false
        canPlace = false
    end
end)