How do I make it so when 2 players join my game, the round starts?

The Issue? You're not scripting in FE.

What do I mean by FE? FE stands for Filtering Enabled, the type of client-server system Roblox has used on all games since 2018. It's a game industry standard, meant to help minimize exploits. Essentially, the system is like this:

Any change on the client stays on the client, it doesn't change on the server. Any change on the server, though, will make the change on all clients, too. The server cannot touch anything ONLY on the client, however.

As a result, any scripting revolving the client cannot be scripted from a regular script. Assuming this is in ServerScriptService or Workspace (I personally prefer the former), it's not working because you're trying to change the UI on the server. UI is part of the client, so the server doesn't have "access" to it. Therefore, using StarterGui won't do anything on the server. Not to mention that StarterGui wouldn't be how you change the GUI. In order to change UI, you need to use a RemoteEvent. This fires an event down to the client, which can then receive it and change the UI if needed.

This is what'd happen when scripting while respecting FE:

--SERVER SCRIPT
--Create a remote event and place it in replicatedstorage
game.local Players = game:GetService("Players")
 local P = game.Workspace.Play --Capitalization is key!

Players.PlayerAdded:Connect(function(player)
    print(player.Name .. " joined the game!")
    P.Value = P.Value + 1
end)
  Players.PlayerRemoving:Connect(function(player)
    print(player.Name .. " left the game!")
    P.Value = P.Value - 1
 end)
 P:GetPropertyChangedSignal("Value"):Connect(function() --No need to change to V, as it's the same thing!
 if P.Value > 1 then --It'll run at 2 like that
    game.ReplicatedStorage:FireAllClients() -- Fires a message to all clients
    print("Success")
end  
end)

You'd need a local script, too now! This is assuming if it's in starter gui:

local PlayerGui = game.Players.LocalPlayer:WaitForChild("PlayerGui") -- This is the actual UI you want to change, Starter GUI replicates itself into the player
game.ReplicatedStorage.RemoteEvent.OnClientEvent:Connect(function() --How to detect event on client
game.Workspace.Beep:Play()
    PlayerGui.ScreenGui.TextLabel.Text = "The party is about to start..."
    wait(2)
    game.Workspace.Beep:Play()
   PlayerGui.ScreenGui.TextLabel.Text = "Don't scream too loud, you'll wake him up."
    wait(2)
    game.Workspace.Beep:Play()
    PlayerGui.ScreenGui.TextLabel.Text = "But most importantly, have FUN!"
end)

If you need more reference: https://developer.roblox.com/en-us/articles/Remote-Functions-and-Events https://developer.roblox.com/en-us/api-reference/property/Workspace/FilteringEnabled

In line 12 you made it so if there is greater or "equal" to 1 player. Take out the "=" if that's not it please message me back.

Here's a script i'm using for my game:

local playernumber

game.Players.PlayerAdded:Connect(function(p)
    if playernumber == nil then
        playernumber = 1
    else
        playernumber = playernumber +1
    end
end)

game.Players.PlayerRemoving:Connect(function(p)
    playernumber = playernumber -1
end)

function playercount()
    for number,plrs in pairs(game.Players:GetPlayers()) do
        if number == nil or plrs == nil then
        return 0
        else
        return number
        end
    end
end

while wait(1 + math.random()) do
    if playercount() >=2 then
        print"ok game starting"
        break
    else
        print"you need 2 players to start"
    end
end

if it works accept , thank you.