Laravel : Class controller does not exist

Add Front part to:

Route::get('plan', 'Front\PlanController@PlanActivity')->name('plan');

Also, change the top of the controller to:

namespace App\Http\Controllers\Front;
use App\Http\Controllers\Controller;
use Illuminate\Http\Request;

And run composer du.

From the docs:

By default, the RouteServiceProvider includes your route files within a namespace group, allowing you to register controller routes without specifying the full App\Http\Controllers namespace prefix. So, you only need to specify the portion of the namespace that comes after the base App\Http\Controllers namespace.