SiteDoors is not a valid member of Workspace "Workspace"?

--[[
	Author: WelpNathan
	Date: 31/05/2018
	
	Script: This script will control how the SCP reacts to touching
			and other interactions.
--]]

---------------------------
--   V A R I A B L E S   --
---------------------------

-- ROBLOX Services
local players = game:GetService('Players')
local physicsService = game:GetService("PhysicsService")
local pathfindingService = game:GetService("PathfindingService")

-- Locate the physical SCP
local this = script.Parent.Parent:WaitForChild('PhysicalSCP')

-- Store the settings of the SCP
local settingsSCP = {
	magnitudeMax = 500,
	sound = {
		damageSound = 1542642349,
		standbySound = {}
	}
}

-- Store the previous weld in memory
local previousWeld = nil

local scpBreached = false

---------------------------
-- M A I N   S C R I P T --
---------------------------

script.Parent.BreachSCP.Changed:Connect(function(value)
	scpBreached = value
end)

--[[
	Function:
		Usage:
			- When called, it will loop through the parts in the model
		  	  and will generate a weld to weld them all together.
		Arguments:
			- model:
				- This is the model which will be welded together. Only
				  supports singular for loop. Won't go into other models.
	
--]]
local weldModel = function(model)
	local parts = model:GetChildren()
	for _, part in pairs(parts) do
		if part:IsA("Part") then
			if previousWeld ~= nil then
				local weld = Instance.new("Weld")
				weld.Part0 = previousWeld
				weld.Part1 = part
				weld.C0 = previousWeld.CFrame:inverse()
				weld.C1 = part.CFrame:inverse()
				weld.Parent = previousWeld
				part.Anchored = false
			end
			previousWeld = part
		end
	end
end
-- Weld the humanoid parts
weldModel(script.Parent.Parent.PhysicalSCP)

--[[
	Make sure that the SCP cannot collide with doors.
--]]
local scpGroup = "GroupSCP"
local doorGroup = "GroupDoor"
 
physicsService:CreateCollisionGroup(scpGroup)
physicsService:CreateCollisionGroup(doorGroup)

function addPartToCollisionGroup(root, colissionGroup)
	for _, child in pairs (root:GetChildren()) do
		addPartToCollisionGroup(child, colissionGroup)
	end
	
	if root:IsA('Part') or root:IsA('UnionOperation') then
		physicsService:SetPartCollisionGroup(root, colissionGroup)
	end
end
addPartToCollisionGroup(workspace.SiteDoors, doorGroup)
addPartToCollisionGroup(this, scpGroup)
physicsService:CollisionGroupSetCollidable(doorGroup, scpGroup, false)

--[[
	Function:
		Usage:
			- When called, it will attempt to locate the nearest player
			  model to attack. It will only attack players. SCPs are
			  allies.
		Arguments:
			- positionSCP:
				- This is the current position of the SCP. This is usually
				  the Torso found in PhysicalSCP.
--]]
local findNearestAttack = function(positionSCP)
	local currentPlayers = players:GetPlayers()
	local pos = settingsSCP.magnitudeMax
	local attackPart = nil
	for _, player in pairs(currentPlayers) do
		-- Check to see if there is a valid model of the player
		if player.Character and player.Character:FindFirstChild('Humanoid') and player.Character:FindFirstChild('HumanoidRootPart') and player.Character.Humanoid.Health > 0 then
			local distance = player:DistanceFromCharacter(positionSCP)
			if distance < settingsSCP.magnitudeMax and distance ~= 0 and distance < pos then
				pos = distance
				attackPart = player.Character.HumanoidRootPart
			end
		end
	end
	
	return attackPart
end

--[[
	Function:
		Usage:
			- When called, it will scroll through a table and see
			  if there is a matching value.
		Arguments:
			- tablePhysical: The table.
			- tableValue: The table value we're trying to find.
--]]
local searchTable = function(tablePhysical, tableValue)
	for i, v in pairs(tablePhysical) do
		if v == tableValue then
			return i
		end
	end
	return nil
end

--[[
	Touch Event:
		- When this is fired, it will proceed to check if a
		  player has touched the SCP. If a player has, they will
		  take damage.
--]]
local damageDebounce = {}
this.BoundarySCP.Touched:Connect(function(hit)
	if scpBreached == false then return end
	local player = players:GetPlayerFromCharacter(hit.Parent)
	if player and player.Character and player.Character:FindFirstChild('Humanoid') and player.Character.Humanoid.Health > 0 then
		if searchTable(damageDebounce, player.UserId) == nil then
			-- Take damage from the player
			-- Add a debounce so they don't constantly take damage
			-- Remove the debounce
			player.Character.Humanoid:TakeDamage(20)
			table.insert(damageDebounce, player.UserId)
			
			-- If there is a damage sound, play it
			if settingsSCP.sound.damageSound ~= nil then
				local sound = Instance.new('Sound', this.Torso)
				sound.SoundId = 'rbxassetid://' .. settingsSCP.sound.damageSound
				sound.Volume = 1
				sound:Play()
				spawn(function()
					repeat wait(3) until sound.Playing == false
					sound:Destroy()
				end)
			end
			
			wait(0.5)
			local index = searchTable(damageDebounce, player.UserId)
			table.remove(damageDebounce, index)
		end
	end
end)

-- Make the SCP into a production environment (remove red)
for _, part in pairs(this:GetChildren()) do
	if part:IsA('Part') then
		part.Transparency = 1
	end
end
this.Torso.Transparency = 0

local destroyed = function(x)
	if x.Parent then return false end
	local _, result = pcall(function() x.Parent = x end)
	return result:match("locked") and true or false
end

--[[
	While Loop:
		Every 3 seconds, it will compare all distances of players and
		if the player is within the SCP's reach, it will attempt to move to it.
--]]
while true do
	while scpBreached do
		local targetPlayer = findNearestAttack(this:WaitForChild('Torso').Position)
		if targetPlayer ~= nil then
			-- Calculate the path and get the waypoints
			local start = this:WaitForChild('Torso')
			local path = pathfindingService:FindPathAsync(start.Position, targetPlayer.Character.HumanoidRootPart.Position)
			local waypoints = path:GetWaypoints()
			
			-- Loop through all of the waypoints in the path
			for waypointIndex, waypoint in pairs(waypoints) do
				if waypoint.Action == Enum.PathWaypointAction.Jump then
					this.Humanoid.Jump = true
				else
					local waypointPosition = waypoint.Position
					-- Make the humanoid move to the current waypoint
					this.Humanoid:MoveTo(waypointPosition)
					-- Wait until the humanoid has reached it's target
					this.Humanoid.MoveToFinished:Wait()
					
					if scpBreached == false then break end
					
					if targetPlayer and targetPlayer.Character:FindFirstChild('HumanoidRootPart') and targetPlayer.Character:FindFirstChild('Humanoid') and targetPlayer.Character.Humanoid.Health > 0 and (targetPlayer.Character.HumanoidRootPart.Position - this.Torso.Position).magnitude < 20 then
						break
					end
				end
			end
			
			if targetPlayer and targetPlayer.Character and targetPlayer.Character:FindFirstChild('Humanoid') and targetPlayer.Character.Humanoid.Health > 0 and targetPlayer.Character:FindFirstChild('HumanoidRootPart') then
				if (targetPlayer.Character.HumanoidRootPart.Position - this.Torso.Position).magnitude < 20 then
					while scpBreached == true and targetPlayer and targetPlayer.Character:FindFirstChild('HumanoidRootPart') and targetPlayer.Character:FindFirstChild('Humanoid') and targetPlayer.Character.Humanoid.Health > 0 and (targetPlayer.Character.HumanoidRootPart.Position - this.Torso.Position).magnitude < 20 do
						this.Humanoid:MoveTo(targetPlayer.Character.HumanoidRootPart.Position, targetPlayer.Character.HumanoidRootPart)
						wait(0.5)
					end
				end
			end
		end
		wait(0.25)
	end
	wait(0.25)
end

Read the error it says sitedoors is not in workspace check if it exist or is correctly named