What Wrong Code ??

PHP:

<?php
//problem help
namespace CountPlayer;

use pocketmine\scheduler\PluginTask;
use pocketmine\plugin\PluginBase;
use pocketmine\event\Listener;
use pocketmine\Player;
use pocketmine\utils\TextFormat as C;

class Main extends PluginBase implements Listener {
    public function onEnable() {
        $this->getServer()->getPluginManager()->registerEvents($this, $this);
        $this->getServer()->getScheduler()->scheduleRepeatingTask(new Timer($this), 60);
        $this->getLogger()->info(C::GREEN."CountPlayer Has Enable !");
}}
class Timer extends PluginTask {

    public function __construct(Main $plugin) {
        $this->plugin = $plugin;
        parent::__construct($plugin);
     }

    public function onRun($tick) {
        $this->plugin->sec--;
            foreach($this->plugin->getServer()->getOnlinePlayers() as $players) {     
                $players->sendPopup("[Game] Start in ".$this->plugin->sec." second(s) !");
                   if($this->plugin->sec === 0){
                    $players->sendPopup("[lGame] Started !! Gogo !");
$this->plugin->getServer()->getScheduler()->cancelTask($this->getHandler()->getTaskId());
                    unset ($this->plugin->sec);


           
}}}}

what wrong my code ???

Mind telling us the Error?

PHP:

public $sec = /*Time*/;


$this->plugin->sec is not initialized.
Also, why are you calling the iterating variable $players? The variable only contains one player each time; that'd make it quite confusing.