Call to a member function middleware() on null when authentication

When I tried to authenticate it gives me this error

Call to a member function middleware() on null

Here is my routes;

Route::prefix('admin')->group(function () {
Route::get('panel',['as'=>'admin_panel', 'uses'=>'AdminController@getPanel']);
Route::get('settings','AdminController@getSettings');
Route::post('save-settings','AdminController@PostSaveSettings');
Route::resource('posts','PostController');
Route::resource('category','CategoryController');
Route::resource('tags','TagController',['except'=>'create']);
})->middleware('auth');

Where do I mistake ?

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