SATE-EasyReceipt/app/Providers/SlackServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;

class SlackServiceProvider extends ServiceProvider
{
    /**
     * Register services.
     */
    public function register(): void
    {
        //
    }

    /**
     * Bootstrap services.
     */
    public function boot(): void
    {
        //追記
        $this->app->singleton(
            'slack',
            \App\Services\Slack\SlackService::class
        );
    }
}