Target class [FrontendController] 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 [Controller] does not exist.

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