"attempt to index nil with UserId" error with data stores?

Hello all, I am in need of support. I have been trying for days to find a solution for this error. Basically, the player variable returns nil, and I am literally unable to find why it's nil. I've scoured the internet for answers but most of them are just specific to the people who made the question... anyways, can someone help me fix this dreadful error?

As always with Data Stores, this is a server script.

local dss = game:GetService("DataStoreService")
local WindowsActivated = dss:GetDataStore("WindowsActivated")

local frame = script.Parent
--local t = frame.activateText
local value = script.Parent.Parent.activation.activated

game.Players.PlayerAdded:Connect(function(plrobj)
	plr = game.Players:FindFirstChild(plrobj)
end)

while true do
	wait(0.2)

	local success, errormsg = pcall(function()
		WindowsActivated = WindowsActivated:GetAsync(plr.UserId.."activated")
	end)
	if not success then
		warn(errormsg)
	end
	if WindowsActivated == "true" then
		--t.Visible = false
		frame.mainScrollFrame.Visible = true
	else
		--t.Visible = true
		frame.mainScrollFrame.Visible = false
	end
end

Thanks in advance.

There is a problem in line 9 and

If there is more than one player (if you are planning to have 1 player per server then you don't need to), the loop won't loop for the other player. Attempt putting the loop inside of the event

Script for more than one player

local dss = game:GetService("DataStoreService")
local WindowsActivated = dss:GetDataStore("WindowsActivated")

local frame = script.Parent
--local t = frame.activateText
local value = script.Parent.Parent.activation.activated

game.Players.PlayerAdded:Connect(function(plrobj)
	local plr = plrobj -- Here is the problem. Before you were trying to do findfirstchild and you put an instance instead of a string. You can also just do plr line 8 instead of plrobj
	while true do
    		wait(0.2)

   		 local success, errormsg = pcall(function()
       			 WindowsActivated = WindowsActivated:GetAsync(plr.UserId.."activated")
   		 end)
    		if not success then
     		   warn(errormsg)
  		  end
   		 if WindowsActivated == "true" then
    	 	   --t.Visible = false
      		  frame.mainScrollFrame.Visible = true
 		   else
      		  --t.Visible = true
    		    frame.mainScrollFrame.Visible = false
   		 end
	end
end)

Script for one player per server

local dss = game:GetService("DataStoreService")
local WindowsActivated = dss:GetDataStore("WindowsActivated")

local frame = script.Parent
--local t = frame.activateText
local value = script.Parent.Parent.activation.activated
local plr
game.Players.PlayerAdded:Connect(function(plrobj)
    plr = plrobj
end)

while true do
    wait(0.2)

    local success, errormsg = pcall(function()
        WindowsActivated = WindowsActivated:GetAsync(plr.UserId.."activated")
    end)
    if not success then
        warn(errormsg)
    end
    if WindowsActivated == "true" then
        --t.Visible = false
        frame.mainScrollFrame.Visible = true
    else
        --t.Visible = true
        frame.mainScrollFrame.Visible = false
    end
end