My script is saying something doesn't exist when it clearly does in the explorer?

Roblox lua doesn't like it when you ask for something when its still loading try this:

	-- Add this below your RandomFloor Variable 
		local RF_Time = RandomFloor:WaitForChild("Time",10) -- wait 10 secs!
		--optional to add this next line below the above line.

		if(RF_Time == nil) then error("Error: Time not found in ",tostring(RandomFloor)," !! ") end

--		replace this:
		Timer.Value = RandomFloor:WaitForChild("Time").Value

-- 		with:
		Timer.Value = RF_Time.Value -- Errors here!

Code in Full:

if EValues.Floors.Value == 10 or EValues.Floors.Value == 20 then
        local NFloors = game.ServerStorage.NPCFloors:GetChildren()
        local RandomizedFloors = math.random(1, #NFloors)
        local RandomFloor = NFloors[RandomizedFloors]
        local CurrentFloor = Instance.new("Folder",game.Workspace)
	local RF_Time = RandomFloor:WaitForChild("Time",10)
	if(RF_Time == nil) then error("Error: Time not found in ",tostring(RandomFloor)," !! ") end
	Timer.Value = RF_Time.Value -- Errors here!
        CurrentFloor.Name = "CurrentFloor"
        RandomFloor:Clone().Parent = CurrentFloor
        print(RandomFloor.Name)
        Timer.Value = RF_Time.Value -- Errors here!
        Audio.Value = 0
        Intermission.Value = 0
        Monitor.SurfaceGui.Floorname.Text = 'Floor Name: '..RandomFloor.Name
        Monitor.SurfaceGui.Reset.Text = "There is a floor that's ongoing at the moment. If you reset to get back to the lobby or just came from the lobby you will not get Coins or Floors."
        wait(Timer.Value)
        Intermission.Value = 1
        EValues.Floors.Value = EValues.Floors.Value + 1
        Monitor.SurfaceGui.Floorname.Text = "Floor Name: None"
        Monitor.SurfaceGui.Floors.Text = 'Server Floors: '..EValues.Floors.Value
        game.ServerStorage.PlayerIntermission.Value = 1
        wait(2.5)
        Elevator.Monitor.Monitor.SurfaceGui.Reset.Text = "It is currently safe to reset to go back to the lobby without losing your reward for next floor. Get back to the elevator when Intermission is about to be over."
        Audio.Value = 1
        CurrentFloor:Destroy()
        Timer.Value = math.random(45, 60)
        wait(Timer.Value)
    end

Hope this helps in debugging this :)