Answers for "Target class [TestController] does not exist."

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');

Target class [UserController] does not exist.

Note: don't forget to import (use) your controller in the routes(web.php) file at the top. Like:
use App\Http\Controllers\PagesController;

Route::get('/',[PagesController::class, 'index']);

Target class [Controller] does not exist.

Route::get('/users', [UserController::class, 'index']);

Target class [SallyController] does not exist.

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

Target class [AppController] does not exist.

public function boot()
{
    ...

    Route::prefix('api')
        ->middleware('api')
        ->namespace('App\Http\Controllers') // <---------
        ->group(base_path('routes/api.php'));

    ...
}