"X is not valid member of Player" - Why does this happen, and how to fix?

So while I was making a Building setup (Mouse localscript, gui, remoteevents, and a script), this error was given to me. Here is the Localscript depended on for creating the blocks, and handles the ghost block:

local Players = game:GetService("Players")
local BuildEvent = game.ReplicatedStorage:WaitForChild("BuildyBuildy")

local localPlayer = Players.LocalPlayer
local mouse = localPlayer:GetMouse()
local Mode = script.Parent.Mode

function roundVector(vector, unit)
    return vector - Vector3.new(vector.X%unit, vector.Y%unit, vector.Z%unit)
end

local hintblock = Instance.new("Part")
mouse.TargetFilter = hintblock
hintblock.Size = Vector3.new(4,4,4)
hintblock.Transparency = 0.7
hintblock.Anchored = true
hintblock.CanCollide = false
hintblock.Parent = game.Workspace

mouse.Move:Connect(function()
    if script.Parent.Mode.Value == 1 then
        local Rpos = roundVector(mouse.Hit.p,4)
        hintblock.Position = Rpos
        hintblock.Parent = game.Workspace
    else
        hintblock.Parent = script.Parent
    end
end)

mouse.Button1Down:Connect(function()
    if Mode.Value == 0 then
        return
    elseif Mode.Value == 1 then
        local pos = mouse.Hit.p
        BuildEvent:FireServer(pos)
        print("Requesting server to build.")
    end
end)

And here is the script in ServerScriptService, depended on for making the blocks appear on demand of the local script:

local BuildingEvent = game.ReplicatedStorage:WaitForChild("BuildyBuildy")

function roundVector(vector, unit)
    return vector - Vector3.new(vector.X%unit, vector.Y%unit, vector.Z%unit)
end

BuildingEvent.OnServerEvent:Connect(function(MouseLocation)
    local newpart = Instance.new("Part")
    local RoundedPos = roundVector(MouseLocation,4)
    newpart.Position = RoundedPos
    newpart.Size = Vector3.new(4,4,4)
    newpart.TopSurface = Enum.SurfaceType.Smooth
    newpart.BottomSurface = Enum.SurfaceType.Smooth
    newpart.Color = Color3.fromRGB(50,225,50)
    newpart.Parent = game.Workspace
end)

Does anyone know where and why the error: "X is not valid member of Player" appears? Thanks in advance.

Edit: Error appeared when I tried to execute the build event, because of roundVector() at ServerScriptService.