SQLSTATE[42000]: Syntax error or access violation: 1055 'auth.orders.created_at' isn't in GROUP BY - laravel-6

Question

Knowing that I made it strict
            
this is my problem;
SQLSTATE[42000]: Syntax error or access violation: 1055 'auth.orders.created_at' isn't in GROUP BY (SQL: select YEAR(created_at) as year, MONTH(created_at) as month, SUM(item_count) as sum from orders group by month)
public function index()
{
$categories_count = Category::count();
$products_count = Product::count();
$users_count = User::count();
$orders_count = Order::count();
$sales_data = Order::select(
DB::raw('YEAR(created_at) as year'),
DB::raw('MONTH(created_at) as month'),
DB::raw('SUM(grand_total) as sum')
)->groupBy('month')->get();
dd($sales_data);
return view('dashboard.index', compact('categories_count', 'products_count','users_count','orders_count','sales_data'));
}//end of index
}
#push('scripts')
<script>
//line chart
var line = new Morris.Line({
element: 'line-chart',
resize: true,
data: [
#foreach ($sales_data as $data)
{
ym: "{{ $data->year }}-{{ $data->month }}", sum: "{{ $data->sum }}"
},
#endforeach
],
xkey: 'ym',
ykeys: ['sum'],
labels: ['#lang('site.total')'],
lineWidth: 2,
hideHover: 'auto',
gridStrokeWidth: 0.4,
pointSize: 4,
gridTextFamily: 'Open Sans',
gridTextSize: 10
});
</script> #endpush