did you try running composer dump-autoload ?

Add the following snippet in your bootstrap/autoload.php file;

/*
|--------------------------------------------------------------------------
| Register The Workbench Loaders
|--------------------------------------------------------------------------
|
| The Laravel workbench provides a convenient place to develop packages
| when working locally. However we will need to load in the Composer
| auto-load files for the packages so that these can be used here.
|
*/

if (is_dir($workbench = __DIR__.'/../workbench'))
{
    Illuminate\Workbench\Starter::start($workbench);
}