Basically I'm building a pet adoption site and now getting an error which says "Target class [admin] does not exist" when my admin user account is trying to approve 'adoption request' of any user who have requested to adopt a pet. What should i do in this case?

kernel.php

<?php

namespace App\Http;

use Illuminate\Foundation\Http\Kernel as HttpKernel;

class kernel extends HttpKernel
{
    /**
     * The application's global HTTP middleware stack.
     *
     * @var array
     */
    protected $middleware = [
        // Existing global middleware
    ];

    /**
     * The application's route middleware groups.
     *
     * @var array
     */
    protected $middlewareGroups = [
        'web' => [
            // Existing web middleware group
        ],

        'api' => [
            // Existing api middleware group
        ],
    ];

    /**
     * The application's route middleware.
     *
     * @var array
     */
    protected $routeMiddleware = [
        'auth' => \App\Http\Middleware\Authenticate::class,
        'admin' => \App\Http\Middleware\IsAdmin::class,
        // Other middleware
    ];
}

AdoptionRequestController

<?php

namespace App\Http\Controllers;

use App\Models\AdoptionRequest;
use App\Models\Pet;
use App\Notifications\RequestApprovedNotification;
use Illuminate\Http\Request;

class AdoptionRequestController extends Controller
{
    // Method to display the adoption request form
    public function create(Pet $pet)
    {
        return view('pets.request_adoption', compact('pet'));
    }

    // Method to store the adoption request form data
    public function store(Request $request, Pet $pet)
    {
        $request->validate([
            'name' => 'required|max:255',
            'address' => 'required|max:255',
            'phone' => 'required|max:255',
            'meeting_date' => 'required|date',
            'reason' => 'required'
        ]);

        $adoptionRequest = new AdoptionRequest([
            'pet_id' => $pet->id,
            'user_id' => $request->user()->id, // Assuming you have user authentication
            'name' => $request->name,
            'address' => $request->address,
            'phone' => $request->phone,
            'meeting_date' => $request->meeting_date,
            'reason' => $request->reason,
            'is_approved' => false
        ]);
        $adoptionRequest->save();

        return redirect()->route('pets.show', $pet->id)->with('success', 'Adoption request submitted successfully.');
    }

    // List all adoption requests (for admin view)
    public function index()
    {
        $requests = AdoptionRequest::with('pet')->get(); // Ensure you load the pet relationship
        return view('admin.adoption_requests', compact('requests'));
    }

    // Approve an adoption request
    public function approve(Request $request, $id)
    {
        $adoptionRequest = AdoptionRequest::with('user')->findOrFail($id);
        $adoptionRequest->is_approved = true;
        $adoptionRequest->save();

        // Send notification
        if ($adoptionRequest->user) {
            $adoptionRequest->user->notify(new RequestApprovedNotification($adoptionRequest));
        }

        return redirect()->back()->with('success', 'Adoption request approved.');
    }
}

web.php

// Admin Routes for Adoption Requests
Route::middleware(['auth', 'admin'])->group(function () {
    Route::get('admin/requests', [AdoptionRequestController::class, 'index'])->name('admin.requests.index');
    Route::patch('admin/requests/{request}', [AdoptionRequestController::class, 'approve'])->name('adoption_requests.approve');
});

IsAdmin.php

<?php

// app/Http/Middleware/IsAdmin.php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;

class IsAdmin
{
    public function handle(Request $request, Closure $next)
    {  
        
        if (!auth()->check() || !auth()->user()->is_admin) {
            abort(403, 'Unauthorized action.');
        }

        return $next($request);
    }
}