您在控制器中将集合命名为“类别”。要在您的视图中使用它，您需要使用相同的名称进行引用。

更改此：

// Your view is looking for a collection titled `main_categories`,which does not exist
@foreach ($main_categories as $category)
  <option value="{{ $category->id }}">{{ $category->name }}</option>
@endforeach

对此：

@foreach ($categories as $c)
  <option value="{{ $c->id }}">{{ $c->name }}</option>
@endforeach

为了更加清楚，我将如下破坏代码并添加一些详细信息

    class CategoriesController extends Controller
{
    public function create()
    {
        $categories = Category
                  ::orderBy('name','desc')
                  ->where('parent_id',NULL)
                  ->get();
// Your are passing variable but I change it as below to be more clear

     //   return view('admin.category.create',compact('categories'));
//now you are passing the value to the view
          return view('admin.category.create',['categories' => $categories]);
    }
}

现在让我们在视图中捕获它。请注意，现在$ categories在视图中可用。如果您通过['A' => $categories]，则视图具有$A变量，因此您应该调用在控制器中定义的相关变量。

   <div class="form-group">
        <label for="exampleInputPassword1">Parent Category</label>
        <select name="parent_id" class="form-control">
**{{-- In here you should pass $categories --}}**
            @foreach ($categories as $category)
                <option value="{{ $category->id }}">{{ $category->name }}</option>
            @endforeach
        </select>
    </div>