Target class [DebtorController] does not exist code example

Example 1: Target class [Controller] does not exist.

//accessing any function in the controller (this is web.php file)
use App\Http\Controllers\PostsController;

Route::get('/post', 'App\Http\Controllers\PostsController@index');

Example 2: Target class [SallyController] does not exist.

//In laravel 8 uncomment  protected $namespace='App\\Http\\Controllers';