Before queueing notifications you should configure your queue and start a worker.

First – Make Migrate php artisan queue:table

after make migration create a new migration file …. 2019_05_13_163626_create_jobs_table.php

Make command : php artisan migrate

Config Queue Driver :  open , .env and config like this (more connection driver in config/queue.php)

QUEUE_CONNECTION=sync => QUEUE_CONNECTION=database

Running The Queue Worker : php artisan queue:work

App\Mail\DemoEmail.php

class DemoEmail extends Mailable implements ShouldQueue
{
use InteractsWithQueue, Queueable, SerializesModels;

}

Show More : https://laravel.com/docs/5.8/notifications