After installing fresh version of laravel I was getting the above mentioned error. I have done 'composer dump-autoload' and cross checked the paths and everything seems to be correct. I am sure that everyone is going to face the exact issue. So please check my answer below, it will help you.
Thank you
The issue is very well explained in the upgrade guide. But for your quick reference I will post it here.
In previous releases of Laravel, the RouteServiceProvider class contained a $namespace property with a value of App\Http\Controllers. This value of this property was used to automatically prefix controller route declarations controller route URL generation such as when calling the action helper.
In Laravel 8, this property is set to null by default. This allows your controller route declarations to use the standard PHP callable syntax, which provides better support for jumping to the controller class in many IDEs:
use App\Http\Controllers\UserController;
// Using PHP callable syntax...
Route::get('/users', [UserController::class, 'index']);
// Using string syntax...
Route::get('/users', 'App\Http\Controllers\UserController@index');
In most cases this won't impact applications that are being upgraded because your RouteServiceProvider will still contain the $namespace property with its previous value. However, if you upgrade your application by creating a brand new Laravel project, you may encounter this as a breaking change.