In RouteAction.php line 84: Invalid route action

If you are using laravel 8 you need to add your controller and method name inside the array, otherwise, it will throw an error.

Route::get('/projects', User\ProjectController::class, 'index')->name('user.projects');

TO

 Route::get('/projects', [User\ProjectController::class, 'index'])->name('user.projects');

I also face a similar problem:

<?php

/*
|--------------------------------------------------------------------------
| Web Routes
|--------------------------------------------------------------------------
|
| Here is where you can register web routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| contains the "web" middleware group. Now create something great!
|
*/

Route::get('/', 'Frontend\FrontendController@index')->name('home');
Route::get('/post', 'Frontend\FrontendController@post')->name('post');
Route::get('/contact', 'Frontend\FrontendController@contact')->name('contact_us');
Route::group(['prefix' => 'admin'], function () {

    Route::get('/create', 'Backend\BackendController@index');

    //User Route

    Route::get('/registration', '');
});

And I just remove the Route::get('/registration', ''); and it's work for me :)

Those who are new to Laravel or learning use

Route::resource('resource_name','controller_name')

to avoid this kind of error when you type:

php artisan route:list

In cmd or any other command line.

Because it is invalid. As you're using GET route, you must specify method name(unless you used ::resource):

$this->get('dashboard', 'DashboardController@methodName');