__clone method called on non-object why aim getting this error when I am trying to get user id from event

I am trying to send a notification to the enrolled users in an event a day before the event’s start date. I have a user table events table that contains the start date of each event. Also, I have an enrollment table that includes the user_id and event_id as foreign keys and the start date of these events. However, while I was trying to code the logic of this function and executing it, I got this error.

<?php

namespace AppConsoleCommands;
use AppModelsEnrollment;
use IlluminateConsoleCommand;
use AppModelsUser;
use AppModelsevent;
use IlluminateSupportFacadesDB;
use CarbonCarbon;
use IlluminateNotificationsNotifiable;
class SendNotification extends Command
{
    /**
     * The name and signature of the console command.
     *
     * @var string
     */
    protected $signature = 'send:notification';

    /**
     * The console command description.
     *
     * @var string
     */
    protected $description = 'send automatic notification to users that enrolled in events a day before the event date';

    /**
     * Create a new command instance.
     *
     * @return void
     */
    public function __construct()
    {
        parent::__construct();
    }

    /**
     * Execute the console command.
     *
     * @return int
     */
    public function handle()
    {
      //  $users = User::all();
       // dd(count($users));

        $enrolls = Enrollment::all()->keyBy('user_id')->all();
    
       $events = DB::select('SELECT start_date , user_id FROM events , enrollments where events.id= event_id');
        
         

                foreach($events as $event) {
              
                $created = new Carbon($event->start_date);
                 $now = Carbon::now();
                 if((($created->diffInHours($now))>=24 )&& (($created->diffInHours($now))<=48)){
//$user = User::where('id', $event->user_id['user_id'])->first();
                $user = User::where('id', isset($event->user_id))->first();
             // $user = DB::select('SELECT * FROM users where id='.$event->user_id);
//dd($user);
                 $user->notify("Your deadline is in  day!"); 
                 }
            }        
        
    }
}