Why is CharacterCustomizationUI not a valid member?(FE)

I have this script that should weld a part to the player when the remote event is fired. This works in studio, however when I test it in a server, I get this error. Is it because I'm trying to access the players GUI from the server?

00:17:29.082 - CharacterCustomizationUI is not a valid member of PlayerGui

-- Server Script -- 
function weld(p1,p2,c0)
    local w = Instance.new("Weld",p1)
    w.Part0 = p1
    w.Part1 = p2
    w.C0 = c0 or p1.CFrame:inverse() * p2.CFrame
    return w
end

function PerformWeld(player)
local DataLoad = player.Character:WaitForChild("PlayerData")    
local PlayerAttire = DataLoad:FindFirstChild("AttireData")
local WD = DataLoad.WeldingAids:FindFirstChild("WeldData")
local Weldee = DataLoad.WeldingAids.Weldee
local PE = player.Character:FindFirstChild("PlayerEquipment")

local Outfit = game.ServerStorage.GlobalStorage.Attire:FindFirstChild(Weldee.Value):Clone()
    local char = player.Character or player.CharacterAdded:wait()
    local Garment = Outfit:WaitForChild(WD.Value.."Components")
    local CurrentSection = Garment:Clone()
    CurrentSection.Parent = char
    local H = Instance.new("StringValue")
    H.Name = ("Hair")
    H.Parent = CurrentSection

    for _,v in pairs(CurrentSection:GetChildren()) do
        if v.ClassName == "Part" or v.ClassName == "UnionOperation" then
            v.Anchored = true

        end
    end

    CurrentSection.Parent = PE

    for _,v in pairs(CurrentSection:GetChildren()) do
        if v.ClassName == "Part" or v.ClassName == "UnionOperation" then
            if v ~= CurrentSection["ArmorCore"] then
                weld(v, CurrentSection["ArmorCore"])
            end
        end
    end

    local CurrentSectionWeld = weld(CurrentSection["ArmorCore"], char[WD.Value], CFrame.new())

    for _,v in pairs(CurrentSection:GetChildren()) do
        if v.ClassName == "Part" or v.ClassName == "UnionOperation" then
            v.Anchored = false
            v.CanCollide = false

            --print ("Welded "..WD.Value)
        end
    end
end


function DescendList(player)
    local DataLoad = player.Character:WaitForChild("PlayerData")
    local WD = DataLoad.WeldingAids:FindFirstChild("WeldData")  
    local Weldee = DataLoad.WeldingAids.Weldee
    local Hat = player.Character.PlayerEquipment.FakeHeadComponents
    Hat:Destroy()
    local Helm = DataLoad.AttireData.OutfitHelmet
    local T = player.PlayerGui:WaitForChild("CharacterCustomizationUI")
    Helm.Value = T.CurrentFrame.SelectedTrim.Value
    Weldee.Value = Helm.Value
    WD.Value = ("FakeHead")
    PerformWeld(player)

    print("Trimmed")
end

local event = game.ReplicatedStorage:WaitForChild("HaircutEvent") -- make sure the event in in ReplicatedStorage
event.OnServerEvent:connect(function(player) -- OnServerEvent automatically passes the player who fired it as the first argument.   
DescendList(player)

end)

--Local Script--
local player = game:GetService("Players").LocalPlayer 

script.Parent.MouseButton1Click:connect(function()
    script.Parent.Parent.SelectedTrim.Value = script.Parent.Haircut.Value
    player.Character.PlayerData.AttireData.OutfitHelmet.Value = (script.Parent.Haircut.Value)
    local e = game.ReplicatedStorage:WaitForChild("HaircutEvent")
    e:FireServer()

end)

Thanks for any help!