Call to a member function addDays() on string

I'm trying to addDays to a date using Carbon with Laravel, but I don't get why I'm receiving this error. I have checked some posts in StackOverflow, but no one has helped me to solve it.

This is my code:

$user = Auth::user();
    $orders = DB::table('orders')->where('user_id', Auth::user()->id)->get();
    $totalValue = 0;
    $percentage = 0;
    $currentDate = $carbon->now();
    $currentDate = Carbon::parse($currentDate)->format('d/m/Y');
    //$percentage = ($order->price * 0.05) / 30;


    foreach($orders as $order) {
        $nextDate = Carbon::parse($order->created_at)->format('d/m/Y');
        if(1 == 1) {
            $nextDate->addDays(1);
            DB::table('orders')->where('user_id', Auth::user()->id)->update(['profit' => $order->profit]);
    }

The error:

via Chebli Mohamed