How do I fix this?

I see by your coding style, you are setting up your game to fail in the long run. As an example, I am going to provide you some code which:

1. Could improve your coding style.
2. Create a more secure game.

local PlayerManager={} do
	PlayerManager.RegisteredPlayers={};
	local Methods={};
	local Metatable={__index=Methods};
	
	function Methods:Destroy()
		PlayerManager.RegisteredPlayers[self.UserId]=nil;
	end;
	function Methods:IsPlaying()
		return self.Data.IsPlaying;
	end;
	function Methods:ToggleIsPlaying()
		self.Data.IsPlaying=not self.Data.IsPlaying;
		return self:IsPlaying();
	end;
	
	local function RegisterNewPlayer(Player)
		local Data={};
		Data.IsPlaying=false;
		Data.Score=0;
		Data.UserId=Player.userId;
		
		return setmetatable({Data=Data},Metatable);
	end;
	function PlayerManager:Get(Player)
		local RegisteredPlayer=self.RegisteredPlayers[Player.userId];
		if (not RegisteredPlayer) then
			RegisteredPlayer=RegisterNewPlayer(Player);
			self.RegisteredPlayers[Player.userId]=RegisteredPlayer;
		end;
		
		return RegisteredPlayer;
	end;
end;

local NewPlayer;
game:GetService("Players").PlayerAdded:connect(function(Player)
	NewPlayer=PlayerManager:Get(Player);
	print("New player ".. Player.Name .." has joined the game!");

	if (GameEventIsOccuring) then -- example
		NewPlayer:ToggleIsPlaying();
	end;
end);
game:GetService("Players").PlayerRemoving:connect(function(Player)
	PlayerManager:Get(Player):Destroy();
end);

If you have any questions or concerns on what my code does, feel free to